Hi all, sorry for long time no post. Happy to say that since last post my windows have stayed up and the car has stayed locked. I don't live near any military or police stations, and as far as I know nothing out of the ordinary is around the area I live.
The Citroen key fob has magically started working normally again. The unlock button works every time, just like it used to.
It seems too much of a coincidence to me that when that was playing up, was around the time the Leon's windows went down and the car unlocked. The C2 is due in for service soon so I will ask the Citroen dealer to check the key fob and tell them what's been happening.
Still doesn't explain why my neighbour's Honda key fob appeared to have unlocked the Leon once, though - but then I can't prove that for sure.
All very odd, and something I'll keep a close eye on and mention to the dealer next time I'm there.