Sad but wise policy given recent events in Manchester etc: Priddy Folk Festival (7-9th July) have instigated a policy of searching bags taken into venues. see Priddy Folk Festival security page

Of course it may reveal who brings their own instead of buying booze at the bar.

AND I might add - lock your valuables in the car, or put them in the sleeping bag with you - like your wallet.
The first night is usually the critical time when people are inebriated, elated and glad to get some kip. You can't predict which festival will be targeted, but faces in the tent opening saying "Sorry wrong tent" are a good indictation - take their photgraph if you can.
An unzipped tent in the morning - check your trousers/handbag.
Trip to the loo (even in daylight) - hide the valuables first.

My recommendation is to tie Morris bells on the zip - but if it is too windy - use a long enough string so that only an open flap will move them. One guy laid upturned bottle tops along the entrance after he was awakened by a face in the entrance.