The smoke detectors are most likely connected to a breaker that won't normally be turned off by a homeowner. Typically it is wired to the furnace circuit. Open up the panel, it should be the only breaker with 2 hot wires connected to the breaker.

Note: items like furnaces have service switches at the unit to prevent anyone from powering up the device without the service personnel knowing about it. This is true for remote AC units as well.

Dan