Sadly, the rifling of the front of tents is a common practice at festivals and some car shows I go to which tend to have a "younger" crowd. The standard thieves response if challenged is "shorry mate, bit pissed - thought it was my mates tent".
The safest plan is to have everything valuable in the car and sleep with the keys under your pillow/in your sleeping bag :-/
I can cheerfully report however, that one individual recently discovered doing the rounds at Santa Pod last year was soundly giving an arse kicking before authorities arrived