Fly frustrations

Ah summer time. Lovely hot weather, the windows wide open at home to let in any stray breezes that might just be passing by... and the flies. Not so nice. Big fat flies buzzing around the lounge so off I go to get the fly spray from the kitchen. When I get back – no flies in sight, not a single buzz. I wait, fly spray poised. Nothing. Back to the kitchen, put the fly spray away; lo and behold the flies are partying in the lounge again. Retrieve fly spray – they disappear. Do they have an inbuilt imminent-death alert system?