Personally I have never experienced any theft at Le Mans - but I was married to a copper for 10 years and have a bit of an OCD thing about security.
I always lock valuables in the car boot when away from my pitch for any length of time. During the start and finish I join the tent zippers with cable ties. I have even devised an alarm using a rape alarm and a tent peg fixed to the tent zip - just need to remember it is there when you get back.
OK - thieves will probably carry knives which can cut through the cable ties - but they are just as likely to pick another tent with easy access.
Not very romantic but my OH and I sleep at the sides of the tent and keep our clothes bags in the middle between us.
The problems I have seen have often stemmed from thieves taking advantage of drunk people where their beer mask prevents them from being observant about their surroundings or taking care with possessions. I don't often drink*, so will tend to keep an eye on others in our group that are a bit worse for wear.
Debs
*except for the one occasions at the drivers' parade where I failed to specify I needed a glass of wine at the restaurant and got a whole bottle and ended up with a temporary road sign as swag.