Question for UK residents/ tourists in population centres...

I am not sure if it is because I am getting more paranoid due to the 2012 and olympic meme-plex but I have a question for the city folk of the UK...

I have been mooching around Bristol in the sun recently and have noticed, down residential streets as well as on main through-fares, a increasing number of hermetically sealed vans and 7.5 ton lorries. All have thick, rubber sealed, windows, what look like similarly sealed and heavy duty doors at the back (airlock/ HAZMAT design)...all different makes and models (some are very old but the mods look new), all unmarked, all unattended...

Has anybody else noticed these? I am planning to go and get some photos at some point but would like to know if I am alone in this...or not.

Pretty sure they arent meth labs btw!

Cheers.