Update: Thanks for all the answers, some more useful than others. Conclusion is: I went to HR and told them my concerns about said employees behavior. Turns out I wasn't the first to complain so they continued researching his behavior. I did not hear the full conversation but as it turns out the behavior he displayed in our company was the same behavior that got him fired from the last company.
Recently a new colleague started at our company (Germany, approx. 30 employees). Since he started, almost every day right after he comes in he leaves again to get a coffee at subway without stamping into break time. Also he often comes late to work and leaves early.
I am not his supervisor nor are we in the same department. Our offices just happen to be right next to each other.
Thing is: Spotting him coming late to work is easy. Our work plan states he's supposed to come in at 8 AM. I'm in around 7:30 AM so its easy for me to see when he does show up. Often not until 8:15 AM. Tricky part is him not stamping into break time. I can only see his stamped time because I work in the IT department and therefore have access to every employees personnel number via database. So while he's out getting coffee I log into his time account using his credentials to check if he's stamped out. He never is.
Also he pretty often uses his private cellphone in the office which is not permitted but I'm the only one around to notice.
Only him coming late to work is provable by comparing his work plan with his actual work-hours. I can't prove he uses his cellphone or does not stamp into break time unless someone else catches him.
How should I report this behavior to HR? Especially the part with me logging into his time account.
Update: Every employee can look into every time account using the personnel number. There is no policy that forbids one to do so. I'm guessing I stepped in some kind of gray zone. Having access to every number just makes it easier for me, though probably not less ethically questionable.
Update: The work-environment is indeed a call center.