Trouble with Smoke Detectors?

[Craig James Conrad]

I rent out a 1BR Mobile Home in which we have two FirstAlert "Smoke and CO" alarms -- one in the living area, one in the bedroom.  Both are on the ceiling.  These are 1.5 years old with new batteries.  When I first bought this place,  was an LP heater in the living area at one time, so the detectors include the CO option.  When in there several months after the tenant moved in, I saw that he had taken the bedroom detector down and removed its batteries.  When I asked about it, he said that it would often alarm when he took a shower.  Assuming the unit might be bad, we swapped the two alarms.  He reported the same behavior (on the unit that was now in the bedroom). 

I figured it must be set off by the humidity of the shower, especially as the bathroom lacked a vent (it is an older mobile home).  So, I installed a powerful bathroom vent in the bathroom ceiling, and reinstalled the alarm.  In the mean time, I removed the LP heater and the leased LP tanks, so there is no more gas used in the place (so the CO part of the alarm shouldn't be an issue) and installed a mini-split heat pump in its place.

I hadn't heard anything from the tenant but, when they moved out last week, I saw that they had again disconnected the alarm in the bedroom. I asked and they said it still would go off for no valid reason.

While prepping the unit for the next tenant, I replaced the batteries and re-installed the alarms. Last evening (while the unit was empty) that alarm started going off and a neighbor called me.  I went in, disabled it, and could find no reason for the alarm.  The unit was about 60 degrees, nothing burning, no odors, no running water to create steam -- nothing.  I brought the alarm to my house and set it in our hallway and it hasn't gone off at all.

What could be causing it to go off (remember that, according to the tenant, another identical unit was also going off when installed in that same bedroom).

Thanks!