Shhhhh! Don't tell everybody! (It's noisier than the Cess.... aarrgghhhhhhhh..........
Some years ago I could hear a constant faint helicopter drone - unlike the usual Met Police EC135 or 145 orbiting round the scene of the crime, no rise and fall in the engine noise or obvious rotor slap. I went outside and could get no sense of its direction, looked up and could see a small dot. Bins identified it as a blue and yellow (ie PAS) Squirrel hovering at 10000' or more somewhere over South Croydon. It was there for quite some time, never seen a Met helicopter do anything similar except briefly at low level. Always wondered what he was doing...