Yes, we too experienced our tent 'tagged' for the first time this year on a site we stayed at in Newquay. Didn't bother us though, but did have a laugh at the (same) guy that came round to check our tag every morning; we were there for two weeks!
The op mentions French sites & certainly theft has been a problem from these with outsiders targeting sites. Its actually good to see the large French sites in popular holiday areas are taking security of their paying customers more seriously these days with uniformed security guards day & night & the visible wrist bands worn by campers while irksome at least makes it clear to security who should be on site & who should not.