I have 3 Kidde i2020 smoke alarms. They are AC powered with no battery backup. I had several power failures last weekend and now all 3 smoke alarms are very intermittently chirping at me. They are not interconnected units (I believe, as they do not all alarm when I press the test button) so I don't think it's just one unit being a nuisance. They don't seem to chirp at any sort of interval, but it happens at least once every half hour from each unit. Any ideas? They are dated 2007 so they shouldn't be expired. I have tried the silence button which works great... for about a half hour.