I think he was being serious... unfortunatly if it looked like easy pickings its not unheard of for them to wait til they think you've had stuff replaced on insurance and do it again.
Ah in which case sorry, police didn't specifically bring it up but I'm aware that they might return. Hence why I was asking about Burglar bars etc. It'd be considerably more difficult to get past them than a wooden window frame.
I'd be leaving booby traps for the scumbags, regardless of legality!