Put an IR LED on the robot modulated at 40 kHz (perhaps us a 555 timer). The LED is on when the power is on. Put an IR receiver module (like used with remotes) in the cabinet and when that receiver detects an "on" robot signal (the modulated IR) the receiver drives a loud piezo alarm.
Here is a simple circuit using a small relay that will disconnect the circuit when no light hits the photocell -
The photocell and R1 act as a voltage divider controlling the NPN transistor. When light shines on the photocell, the transistor energizes the relay and the circuit is powered. When the mouse is placed in a dark place like the cabinet, the relay is no longer energized, cutting off the power. The relay needs to work with your battery voltage. I used an Axicom FP2D3009 5v non-latching relay. Its about the size of a 10-pin DIP chip. If you have 3 battery cells a 5V relay should work.
The light switch consumes 25mA when active and around .02mA in darkness. In total darkness, the current draw was too small to measure (ohm's law - 5V / 5M Resistance =3D .0001mA). If there is partial light, the circuit will consume more in the off state.
I'm sure there are many other ways to do this but this was the least part count I could come up with.
Eddy Wright Wright Hobbies Robotics
The mouse will shut off in any dark spot so if it scurries under the couch, it will shut off. You can adjust R1 to change the darkness shutoff threshold. Increasing R1 reduces the darkness sensitivity. I use 10k and 100k values and both worked fine.
Parts: Relay - Axicom FP2D3009 5V or similar PhotoCell - 2M in Darkness, 1K in full light Q1 - 2N3904 or equivalent R1 - 10K to 100K or higher