This might make me sound paranoid, but hey...
When I first started airsoft a few years ago I normally went on my jack, I went to a site that I'd never been to before, so didn't know the layout or the people.
Needless to say, I wanted to make sure I was fully prepared to keep my very expensive AEG's (at the time) & gear secure in the safezone, so I went down to screwfix and bought one of those heavy duty motorcycle loops, along with some Abus combination locks, I tie my rifle case & bags to the leg of the table & use the Abus locks on all my rifle cases & bags so there isn't a single bag left that someone can peep into... Being tied to the leg of the bench makes absolutely sure no one can go walkies with it, as a padlocked bag/ case brings suspicion that there is something definitely valuable inside.
It is unheard of on the sites I play at, but it does happen, and I don't want to fall victim to it.
I do get quite alot of funny looks from the regulars when I pull out this massive bike loop and loop it round all my gear, but hey, I'm not taking any risks.
If there is something that I don't need out, i.e. a backup gun, then I keep it locked in the boot of my car out of view (probably not ideal as the carpark is less watched).