This sort of thing is annoying and upsetting - I was off sick for a week and my desk was forced open, allegedly to get hold of a document I was working on. This was a pointless exercise, as it was available online, and could have been obtained simply by phoning me up to ask where it was kept.
When I complained to my manager, I found that he, together with the responsible line manager, had actually carried out the break-in.
I'm far too old a hand to leave anything personal or confidential in my desk, so there was no comeback on me. A few months later, I got made redundant - unfairly, in my opinion. I'm sure they'd been trying to get rid of me for some time. I now suspect that the break-in may have been an attempt to find something in my desk that shouldn't have been there as an excuse to start disciplinary prceedings.
So - be careful.