Kidde smoke detector without a door or tab...

Some smoke detectors are AC-only, so there is no battery. Dual-power units typically have the battery compartment on the back of the unit, where it attaches to the wall or ceiling.

Most units must be removed from their mounting bracket to change the battery, as the battery cover is on the back.

You rotate the unit about 1/2 turn counter clockwise, and it should come off the wall/ceiling. If it has an AC power connection, you unplug that so you can change the battery more easily.