The Dowco Weatherall guardian cover survives in marine environments. I live in a densely populated beach community wherein an alarm is probably not a bad idea. This one is simple, loud and will probably surprise the shit out of anyone who decides to move my cover around.
A simple pin plugged into the top of the alarm completes a circuit. The pin is connected to a swivel clip that you attach to your bike. The alarm fits in a Dowco integrated pocket, or simply clips to your cover. The lanyard between the two is fairly short and when the pin pops a lot of noise happens.
A calm and controlled robber will smash the alarm fairly quickly. The wind may blow a loose cover enough to set the alarm off. Under normal circumstances, however, I think adding this alarm to my cover is better than not having it there.
I can not wait until I have forgotten it is there and set off the alarm in my face.