"How would you feel if I went through YOUR stuff?" he roared and on and on, then stormed out telling me my behavior was unacceptable and there was no excuse. Not even the fact that I was told by another staff member that he was gone for the day was acceptable.
I wonder if it isn't that he has something specifically to hide, but that maybe other people (not the OP) actually have
been going through his stuff when they go in there to get approved items. From my perspective, if I was not at work and someone took company property off the top of my desk (a stapler, the kleenex box, even a pen that I actually had brought in myself) I wouldn't think anything of it, but if I found out that someone had rummaged through a closed drawer or file when there was no need, I would feel very upset and creeped out, even if there was nothing in there but work-related stuff. (I most likely wouldn't say anything, though.)
I think we all know people who just can't seem to help snooping around. (Again, I am not talking about the OP, who clearly did not do this.) Maybe other employees have been doing this and this guy is sick of it. Or maybe he's worried about what might happen with other people always wandering through his office area, even if their presence is authorized, because by snooping around or for some other reason, injury might happen or something he is responsible for might go missing, either stolen or just carelessly lost.
Not that this excuses his treatment of the OP. If someone is not showing the proper respect to his work area or he has other concerns, then he needs to take that up with the boss. It was entirely out of line to take it out on the OP.
But maybe there is actually some grain of truth in his complaint, at least as regards other employees, and he unfairly took it out on the OP. And in any case, I agree with others who say just act as though nothing has happened.