Our smoke/fire detectors went off in the middle of the night--the one downstairs first, then the one upstairs, with no detectable cause. We opened some windows and the shrieking stopped. What could have caused this?

This morning as we sat at the breakfast table, a bright light flashed above our heads. The light over the table was off, and it didn't come from the other kitchen light. It last only a second. What is going on?