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.
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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