Help identifying an IC - Motorola SC42604P
It's almost certainly a proprietary part, probably an application-specific smoke detector chip.  The part number would either be dictated by the customer, or cooked up in cooperation between Motorola and the customer.

Smoke detectors are small, high-volume, and at least moderately high-tech; this is a combination that leads to ASICs.

For the purpose of making a loud noise, I suspect you want to look at the sounder and the transistor that's just below it in the component-side view.  I suspect that the chip just applies power to that circuit, or possibly bias, or a square wave applied to the transistor base.