The CEL came back the day after my last post with the code P0047. I found this TSB from Ford : https://workshop-manuals.com/ford/crown_...tc_p0455/p0457/
Turns out it was exactly that. The fuel cap strap was trapped between the fuel cap and top of the filler neck causing the CEL to come on.
2003 Mercury Grand Marquis LS - 86k miles
1996 Mercury Grand Marquis LS w/ Handling Package - 20k miles
2005 Mercury Grand Marquis LS Ultimate Edition - Sold at 95k miles
2011 Lincoln Town Car Signature Limited - Sold at 97k miles