our house has hard wired smoke detectors Last night they went off but there was no fire Why?

if your smokes sounded you need to find the trigger unit. after a unit triggers its lights will usually blink indicating it activated. Do you have smoke/carbon monoxide detectors if so check these first for sign of activation. Many different situations can set off false readings. manuals that come with units tell by beeps (short and/or fast) what trouble occurred. If they all went off that was not battery prob, so you can eliminate that as a reason. Low battery sounds short chirps on low battery unit only. If one is in defect mode it can false trigger the others. You need to find the source or replace all for peace of mind. Go to any electrical supply house and ask them for assistance they deal with the pros's

smoke detectors are hardwired and also have a battery back up. They will go off when the battery starts to die or there is some blip in the electricty to the detector.

It may have been the circuitry, or possibly a power outage that was so short only the smoke detector detected it.

if there was a sudden rise in temperature it will make them sound off or if someone walked past or stood near it with a cigarette it will go off. also you may have a carbon monoxide detector built in with it, if so you should find out where this gas is coming from.

If your windows were open, sometimes the flow of air will cause them to go off. IF, you didn't have your windows open, no clue.

they are full of dust...blow em out with canned air (computer duster)

THEY MAY NEED A NEW BATTERY