I keep our trailer stored in the corner of a fenced in gated storage area next to a HUGE boat. Unfortunately, vegetation has grown up making it partly concealed, so of course someone broke into the boat and in passing, browsed thru our trailer. He got from us:
A badly worn r-dome (without any poles)
A nice 10X12 tarp I use as an awning
11 nice forged tent stakes
He wasn't a vandal, except for damage incidental to perusing our property, he did no further damage. In the excitement (it was dark), the poor wretch left behind in a pillow case several flashlights and a weather radio. We had recently removed our battery, propane tanks and a tool bag filled with lots of goodies while our trailer was in the shop getting axle repair work done, otherwise at least the tools would have been gone, too.
I am mentioning this because it's an object lesson - 'Stuff' happens
Keep your trailer in a well lit area
Keep tools and other valuables out of the trailer when stored.