G the E, I sincerely hope that what you've spotted isn't a pattern. Kids/vandals seem to either do relatively minor damage (nicking fire extinuishers from parked A/C at EGLS springs to mind) or major damage (smashing up all those A/C at EGKR) rather than something as horribly subtle as you describe.
I rather suspect that there cannot be too many individuals that would have such a twisted frame of mind as to do something like this, which rather leads one to suspect a connection between the two events.
Perhaps it is someone with a mindset rather like that aggrieved (and mentally disturbed) cyclist in Bournemouth, who was recently found guilty of slashing hundreds of car tyres in misguided retribution for having been splashed by one once when out on his bike.
It rather behoves us all to very, very thoroughly check for similar damage, and perhaps think about devising some form of unauthorised entry detector to alert us of possible intruders.