The weirdest theft

When I came to work this morning after the weekend, I noticed that my two pens were missing their covers.

Then this afternoon, I was using my pencil (a mechanical one) when I flipped it and two pieces of lead fell out. The eraser at the end was missing. Probably since this morning as well...

What kind of practical joke is this?

(I leave two pens and a pencil on my work desk, since they were given to me at the lab. I used to leave a stack of post-its there for the same reason...until it disappeared one day.)