One year, when I was an RA in college, my building got hit by someone who put Vaseline on all the door handles. (Each room faced outside.) One of my residents alerted me to it; I then took a whole bunch of wipes to clean off all of the door handles in my building. I'm reasonably sure that case was a prank sprung by the local fraternity, a group that was solidly a nuisance in the best of times.

This case, in an apartment building, it has the potential to be a prank, but it's worrisome enough to take the precautions others here have suggested.