Why does my smoke detector chirp when my bathroom lights are turned on?
I don't think I've seen a smoke detector that only chirps when it's light out, but I'd love to get one if that's the case. Mine always seem to chirp at night (typically because it's colder and batteries fail in the cold).

I have seen smoke detectors with a light sensor used in place of a test button, but this typically requires a flashlight shined directly on it.

My guess is that you don't have a battery in there at all and when the lights come on the smoke detector is getting reset and doing an activation chirp. Replacing or installing a battery would fix this. But I would double check that the smoke detector is working when the lights are off, on the off chance someone screwed up and wired it into the switch.

My second guess is that the voltage drop from the lights turning on, combined with a borderline low battery, is enough to cause the low battery chirp to trigger. But this would get worse as the battery dies until it's constantly chirping.