A couple days ago I found out whenever I'm out of the office (meetings, vacation, etc), that one of my coworkers have straight up been using my work computer in addition to their own work computer before logging off and putting everything back to normal before I'm back. I asked them about it and they said that it was okay because they logged into their own account and it is a company computer. That's true I technically don't own the computer but it's the one assigned to me so isn't it expected that others don't use it, especially so secretively? They won't tell me what they're even using it for (though it's probably non-work stuff...) and I am honestly shocked that this has just been going on without me knowing at all. Shouldn't they ask my permission to use my computer first, or is this acceptable behaviour since it's a company computer and they use their account?
I think you're over reacting a bit here.
It is a company computer // I technically don't own the computer
Well then what's the problem?
Surely when you go away the person cannot log into your account (if so, ensure you lock your computer when leaving it), so even if they do use your computer for none work related things it'll be tracked on their account (Assuming activity is logged) if not try and get email proof about the conversations just be like when you used my PC did you.... ?.
As long as it doesn't intervene with your work I suggest you just turn a blind eye and just carry on with your job as it really doesn't and shouldn't affect you. If your manager/whomever it concerns does ask you about activity on the computer you have email proof, or even ask colleagues
If you don't know what they are doing with your computer (or, even worse, if you know what they are doing and it's non-working stuff) you should speak clearly with them and, if they continue, address the issue with your manager.
While it's true that the computer is not yours and any action they take is going to get logged under their account, it's still a company asset assigned to you. You might even have signed some piece of paper stating you'll take care of the company computer (I know I have done it) and you'll be responsible for any non work-related incident involving that computer. Moreover, action (malicious or erroneous) on their part might compromise your ability to work with that computer. This is going to impact your job and thus is clearly something your manager should know about (they may be ok with it, but it's not your job to take that decision for them).
It doesn't matter that much.
This is a work computer, so there shouldn't really be a need for "your" data to be on there - you have an expectation that anything you do on a work computer is liable to be visible to co-workers/IT.
So you lock your workstation and someone else logs in with their own corporate credentials. It seems as though nothing bad has happened, but you feel some kind of ownership over "your" computer.
I feel the same way when someone uses my desk when I'm gone, leaves their chewed biros on my desk, lowers my chair. What do I do about it? Shrug, reset my chair and throw the crap away and get on with my day.
If the configuration of your computer changes (software installed/uninstalled in a way that interferes with your day-to-day work, then address this with your manager or co-worker as appropriate.
They claim that they are using their own account, but you are not sure about this. You don't know what they did so you need to protect yourself but without "going to war" with your coworker.
1) Check with IT about the status of the computer. ( it is likely that it should only be used by you). But before you inform your boss you shoul check the internal rule to avoid going to your boss and find out in the end that you are wrong.
2) After you checked with IT, voice your desagreement during a a face to face with you corworker : "hey guys, please don't use my computer i don't like it". You can also ask why they do that, but don't pressure them, just ask one time casually to see if they reply. You don't want to be the "annoying guy in the office" even if you are right.
3) Have a face to face with the manager : " hey boss, other cowokers used my computer, maybe they did illegal stuff, i don't know, so i think my duty is to inform you". (again, you don't want to be the guy that complain, but be the guy that wants the best for the company and inform his boss when there is something suspicious.). after the face to face, send an email to your boss to sum up what you talk about. so you have a proof : as soon as you know something suspicious you have inform your manager
You don't know what they did For whatever they did, they use your computer ( IP + mac adress) -> you need to protect yourself.
Non-escalation related solutions:
- offer to "help" him in the tasks he performs in your computer, do not be afraid to try to understand why the same task cannot be accomplished in his computer;
- if he doesn't provide a clear explanation, jokingly ask him if he's watching "you know what" when he uses your computer... If he shies away, look to him seriously and tell him that you will not take lightly someone using your assigned computer to other matters than work;
- as another dialog alternative, point out that the keyboard and mouse are vehicles to propagate contagious diseases (they generally have a lot more bacteria than toilet seats) and since you often get sick, you want to avoid to getting sick more times than usual, also, it would be unfortunate that he would start getting as well (let him understand if he's going through all the trouble logging in in your computer, ask him to use his input devices, if he breaks them or makes a mess out of them, you will not be blamed for it and you may consider escalating the issue if he simply does not respect your request)...
To check what he may be doing:
eventvwr(Windows) or some sort of logger to know which time frames your computer was used by other person(s);
- Use some tool for verifying which files were recently deleted (in SSDs it's particularly easy to recover them);
- If your account is an admin of the computer you are using (i.e. the IT department gave you that privilege by default), you are allowed to access his user folder to check temporary/browser files;
- Install a screenshot snapper (again, if you are an admin) and make it run globally.
If you identify files that are non-work related which may be harmful to you if found by others in your company (especially if later you are reassigned), print the logs of both times and files you found and head to management directly and prove that you were not the one using the computer. Let management decide your colleague's behavior (you don't need to tell your colleague anything if this is the case).
I was responsible once for resetting up a laptop after one of my colleagues left the company. My boss at the time had enough lack of curiosity to simply ask me to wipe out his files, since all relevant files to the company were already shared. So I simply needed to do a basic clean and prepare it for a new worker. I was doing that in the beginning, but it was clear that my former colleague did really not care or dedicate any time cleaning his junk files and I ended up wasting more time making sure none of that stuff showed up anywhere else later. The one coming after him would have definitely found those and would perhaps reconsider working there.
Of course depending on your company policy...
Secure your computer
Consider enabling BitLocker (or whatever drive-level encryption is relevant to your OS) so that a password is required to boot the machine. They will need to awkwardly ask you to unlock it if they have a legit reason to use it.
I personally doubt that you could be reprimanded for taking data security seriously with regards to securing your workstation.
Edit: Unless your workplace specifically uses hot-seats, where your computer MUST be accessible by your coworkers.
Secure your individual data
If encrypting the entire drive is not an option, I would still strongly recommend enabling encryption for at least your own user account. As a good practice.
Because there is still the risk that somebody can access your data from another account or even by booting via a live disk like Kali Linux. One of the many purposes of Kali Linux is nefariously stealing data.