Last night we had a smoke detector activation in the middle of the night. No smoke was detected anywhere in the house. The dectectors are direct wire. Does anyone have any ideas what could cause the detectors to activate other than smoke?

Some factors to consider...

The detectors are 13 years old. Weather conditions were unusually cold (-10 F) outside. The house is heated by Geo-Thermal Heat Pump and no fireplaces.

The alarms stopped activating after the main cicuit breaker was intentionally turned off and then back on.

Have you maintained these detectors ever?
Smoke detectors should yearly be cleaned, checked, and maintained by a specialist.