Fluorescent Ejaculating Smoke Alarm

Smoke alarms of my acquaintance beep intermittently when the batteries are running down. This is all very well, but what if you're either deaf or not around when they finally go flat?

Put a riot foam dispenser inside. As the battery runs down, use the remaining power to hold a nozzle shut with an electromagnet, but only deploy this as it runs down to save power. When it finally does fail, the nozzle opens and quick-drying chartreuse fluorescent foam snakes spring forth covering the space 'twixt ceiling and floor, thereby alerting the occupants of the need for a new battery.