Fancy Target

This is the swimming pool roof at Centre Parc.
It is in the middle of 1000 acres of woodland and gets 'buzzed' fairly regularly by the RAF doing their low level training.
Not surprising really because it must by an ideal spot to find on a map.
The problem is, because you are down in the trees you can't see them coming and they are almost on top of you before you hear them which makes it really hard almost impossible to get a photograph as they come over.
Nearly as hard as getting a decent shot of a red squirrel - they are very adept at waiting until you are nearly within shooting range (but not quite) and then bounding off laughing.
What I want to know is, how do they always know what focal length of lens you have?

Speaking of the squirrels - the whole forest is a red squirrel sanctuary and you are asked to contact the staff should you see a grey and they will go and 'deal with it'.