Would definitely agree with Baz above regarding pics....I have a CD of about 150 photos of my instruments (No, that's not 150 instruments, but close-ups/numbers/distinguishing marks etc) Took me a morning, and it made me polish them up too! Only problem was I couldn't work out how to take a picture of the camera that took the pictures!
As for leaving things on a campsite....Would never even contemplate it. Most festivals have some sort of facility for stashing things securely away. Wouldn't even trust a locked car nowadays.
At one particular festival I attend regularly, The B&B I stay in has a room that lots of artists store their instruments in for the duration....It resembles a high class music shop sometimes! (And No, I won't tell you where it is!)
On the very rare occasions I camp, Thieves would find very lean pickings. maybe a half finished bottle of Red, a can of Stella, and a couple of pairs of used underpants....And the tent isn't worth much anyway!