To detect security breaches, Alice and Bob should send a token amount of bitcoin (say 1 BTC) to the public keys that they are using, and should never spend those coins. Both addresses should be monitored by both Alice and Bob, and if they see coins being spent they should assume that the corresponding private key has been compromised and transfer the multisignature coins to a new, secure multisig address with fresh keys generated on devices that have not been compromised.
Unless I'm a rat after the big block of cheese, in which case I'm probably smart enough to pass up the small piece sitting on the rat-trap while making my way to the fridge.