I finally went by the AMP in St. Peters, on Spencer Road just off of Mexico road. I stopped by today (Tuesday) around 2:00. The "Open" sign was turned off, and the door was locked. I came back about an hour later, and nothing had changed.
I don't know if I just caught them at a bad time, or whether this place has gone the way of the Ballwin AMP.
If I find out any more info, I'll let you all know.
