Does your smoke detector ever go off without explanation? If nothing is burning, your shower may be the cause of those alarming sounds.
False Alarm
Steam from the shower can sometimes activate a smoke alarm. Though it looks like a cloud to us, steam is made of airborne particulates—millions of tiny particles of water floating in the air.
Smoke is very similar. What appears to be a dark cloud or haze to our eyes is actually made of millions of tiny particles—in this case, not of water, but of burnt matter.
Ionization vs. Photoelectric
There are two kinds of smoke detectors: ionization alarms and photoelectric alarms. They work in different ways, but both are designed to flip a switch when they come into contact with airborne particles. A sudden cloud of steam may trigger a photoelectric cell looking for particles as quickly as a cloud of smoke.
Better steam than smoke, but you might think about getting a new smoke detector if you don't want any more false alarms.