I thought about a paging alarm, but decided if the siren doesn't concern the thieve(s) then I'm not sure I want to be around anyway.
I can imagine running out to see 4 guys loading my bike into a truck. Am I going to yell at them and shake my fists?
Bolting it to the ground when you can so it's "too much work to steal" and pay your insurance bills is probably best way to go!
Cheers.