According to the map I linked to above, looks like there's now a Phase III to add four more sirens.
They are at:
Campus of Logan College of Chiropractic in Chesterfield (2906)
434 N. Sappington in Glendale (2906)
11804 Big Bend in Kirkwood (2910)
Fee Fee Rd 760' S of Natural Bridge (2910)
The locations in Glendale and Kirkwood are both fire stations and had Thunderbolts in the old system.
The locations listed above make sense as gap filler locations except the Glendale location. I realize its a dense residential area, but, there's a 2910 not half a mile from the proposed site of the 2906! I wonder why its not getting the coverage they expected--unless they're going to move it somewhere else. But I find that unlikely.