[X] Leave a drawing of a Go-kart somewhere obvious, for that sweet foreshadowing.
[X] Move their stuff. Everything, but just a few cm, just so that it feels ... off, without it being clear what is off.
[X] Replace Merlins serious mage stuff with the most childish substitutes you can come up with.
[X] Leave a piece or two of surveilance equipment, so little that it seems obvious there must be more. Let them enjoy the paranoia of what else there is hidden.
-[X] Label the surveillance equipment, if one surveillance equipment is left behind, label it #2. If two, label them #1 and #3.
No serious sabotage, just paranoia fuel, annoyance and some Foreshadowing.