Disappearing towers

Could anyone tell me why this annoying thing happens please. I go to an airport, in this last case Grazzanise AB. There is a tower. I see it on the map, and get a good view of the runway from tower view ( it also happens ot other airports) After a few touch and go's, I shut down for a snack of pork pie,pickled onions and a glass of red wine. When I restart, the tower is gone. It's not on the map and the tower view is from God knows where.Now, if it's the porkpie,OK. The red wine I can live without, but please don't tell me it's the pickled onions 'cos they are just too sweet to live without. Seriously tho', anyone know what causes this ?Russell.