
It seems apparent that these thefts are being done by organised people with a plan. They seem to have tickets like any normal attendee which means that they can be at any festival sold-out or not, and any festival large or small.
The thefts all seem to happen on the first night, when people have the most cash and it seems that they are most interested in cash, though if there's anything else they fancy they'll have that too (perhaps even your beer!). They often dump the stolen wallets and purses nearby, complete with everything except any cash.
Sometimes their method is enter the tent as though it's their own, and if they find someone awake inside who challenges them their response is "sorry mate, wrong tent". While this can of course sometimes be genuine (in the dark, tents look very similar) to stop this happening people need to make security &/or stewards aware of this happening at the time it happens; you might save your friends from being robbed, and next time around someone might save you from being robbed!!
Another method is for them to cut a hole in the side of the tent through which they can grab jeans or coats which might have your wallet or purse and perhaps your phone inside.
To keep your possessions safe try to do as many of the following as possible:-
The instances of theft are still comparatively low, and it's unlikely that you'll be a victim of these scumbags. But if you are unfortunate enough to be robbed and haven't taken a few simple measures to make things harder for the thieves then you'll be kicking yourself.
Remember the golden rule: dont take anything that you're not prepared to lose.