I've recently been to a presentation in a client's office which is located in another city. My manager handed me his laptop, modem and pendrive for the presentation. It was on the last day of the week i.e. Thursday (in our country the weekends are Friday and Saturday), and also last day of submission of a crucial document of mine, so I was pretty much preoccupied and tensed regarding this document submission issue, hence I was very unfocused and absent-minded and as a result, I forgot to check whether all my manager's belongings are ok or not before leaving client's office. Yesterday, during office, I discovered that although I've returned his laptop promptly on Thursday, I forgot to return his modem, which had been inside my bag the entire weekend. I promptly returned it to him, and then he asked me, "Where's my pendrive?" I immediately searched my whole bag, and couldn't find it there. I couldn't remember whether I left it in my client office on Thursday, or in my home, so I told him I'd check for it in my home. After searching my home thoroughly, I still couldn't find it. Now I'm pretty sure I left it in my client's office, and unless an employee of my client found it and was honest enough not to steal it, there is very little chance of getting it back.
Now I'm done for, given my stupidity and lack of responsibility to lose my manager's pendrive which may contain vital information of which I have no knowledge. What should I do now? Should I consider today as my last working day? Normally, my manager never gets mad with anyone nor is he very strict, but this incident is serious and I'm afraid he might not take it lightly and punish me to some extent. The worst I can expect is termination. How can I approach him and explain to him this incident? I'm ok with any punishment as long as it doesn't lead me to termination.
Update: Right after posting this question, I called the client and they told me to wait for some time so that they can search for it. After almost 5 hours, they ringed me back and said they found it. I informed this to my manager and he just expressed his relief. He didn't say anything more to me regarding this. They're currently making an arrangement to send it back to my manager.
I was worried over this whole business because had he had any company sensitive data stored in the pendrive, this incident might not have been forgiven. Also, I saw in the comments section, many people asked me why I thought I may never get the pendrive back from the client. In my country, there are plenty of dishonest and immoral people, and the client in question wasn't even a proper client, rather just a prospective client whom we visited to promote one of our products, hence the presentation to them. I personally didn't find them much impressive, compared to the class of clients we generally use to have, so I wasn't much optimistic about their honesty. I was more than happy to be proved wrong twice the same day, first about my assumption of getting fired and then about the client not returning the pendrive. I'm lucky the situation didn't get worse and I consider this event as my "Lessons Learned".