I occasionally lurk on the urbex forums and it appears that at least some of the regulars there are in fact rentaguards.
It is not a “undercover” type thing either; they are genuinely interested in the history of the buildings they are guarding and their concerns over health and safety are real. For one thing there is fuckloads of asbestos around in older buildings…
the sheer levels of vandalism does also mean a certain amount of friction between urbexers and ravers (also many urbex types have encountered problematic drug users in abandoned buildings (using them as a shooting gallery) who haven’t been the friendliest of folks..
Whilst I don’t want to cast aspersions on anyone involved with the urbex forums, I do expect that if it came to the worst they would be far more willing to co-operate with the Police if a building featured on their site was partied and cops started asking difficult questions.