A standard mousetrap (dead or alive) with a low power radio transmitter attached.
The transmitter "pings" once a minute, and could even relay useful information; temperature is the obvious parameter.
When the trap is sprung, the signal stops. The receiver generates a visual and (optional) audible
The system is "positive-fail" as it will alert on a flat battery too. This is in case the trap is set in an unfrequented area where it might catch a mouse but fail to signal due to a flat battery.