I had something relatively similar to this, but I'll put it here as a reply for reasons that will become obvious:
I came home to my shared student house, went to my room, to find one of the posters upside-down.

I went downstairs to the stoners, and asked WTF?

"Oh yeah", they said, "We were going to move all of your stuff by an inch each day", but in the end we decided just to turn the poster upside-down."
( Je suis un vagabond is an unfunny, up your own arse middle class knob, Tue 9 Apr 2013, 14:52, 1 reply)

In one houseshare, I was often last to go to bed.
Two or three times a week, I'd move all the posters, postcards, etc on the chimney breast up by just a tiny amount.
By the time we came to move out, they were all about a foot higher than their starting places.
No-one noticed and no-one believed me when I told them what I'd been doing.
That was a surprise.
( 4Q just lurking, Tue 9 Apr 2013, 22:53, closed)