ptee, welcome to the Forum!!
If your mechanic used a aftermarket sensor that could be some of your problem. Many have posted, saying that when they went to a OEM part from the dealer the light went away.
Before you do that, I'd suggest you do the key dance, which is: from the ACC position, turn the key to start (do NOT start) and back to ACC 3 times within 5 seconds. In the odometer window you should see a P0000, if the PT is throwing a code. That will tell us what may be wrong. The codes are listed in a sticky, or you can Google it.
Did you leave your lights on when the battery went dead? The other possibility is that your Multifunction Switch is going bad. They are on 3-5 month back order. Please come back and let us know what you find out.
__________________
2008 Silver Steel Metallic, Touring, NA, Sirius; Chromed door handles; lower grille; fog light surrounds; chrome front & rear bumpers strips; Airaid air filter; Bassani duals; Modesty Cover; Autolite AP5263 Platinum Plugs (.040); Crane Plug Wires; NAPA Ceramic pads; Blane hood struts; PTeazer Gas, Brake & Foot Rest Pedals, '05 Rear Step Pad, Stainless Door Sills, Llumar tint, "A" Piller chrome; APC Strut Bar