An admiral's hat which I nicked from a dinner in 1989 and cached in a plant pot outside! I even helped look for it when the brown stuff hit the fan.
However, the damn thing was just too hot to handle. Left it somewhere two days later in a carrier bag complete with regimental calling card. Wish I'd held my nerve, but I was starting to get very senior people hinting at my imminment demise whether it was found or not.
In my space cadet days I went to camp at Lyneham and got one of those cheapy multitool things from the NAAFI (god knows why). It had the multiple screwdriver set as part of it and I thought I'd unscrew the sdign from the door at the bottom of the block corridor. Well pleased with myself I managed to remove all the signs from the male segement of the block and a couple from the female segment over the next few days.
Probably still got them stashed up the loft at home. Never managed to liberate anything more useful.