Why would smoke detectors randomly and simultaneously sound?

Twice this month three of our smoke detectors have sounded. Once it was around midnight and today it was at ll:30 a.m. Only one of these is "hard-wired". The other two are battery- operated. One time all the windows were open and it was a cool night. I thought neighbors had fires in their fireplaces or something. Today is cool, but we had no windows open.