Listen for gulls, crows suddenly getting very vocal about something, or if there are any Feral Pigeon flocks, see why they've taken flight, as it will often be a raptor - and where you are that could turn out to be VERY interesting (I've managed White Tailed Eagle, Buzzard, Peregrine, Sparrowhawk, Kestrel, Osprey and escaped Saker Falcon from my living room window, despite my very urban location in Dundee), though sometimes it can be a tiny dot seemingly miles up that is causing the panic.