Strange. Early models of the S197 Mustangs had a similar situation that was related to the 'smart module'. This is the device that kept 'power on' to various accessories for up to a minute after the key was turned off. Disconnecting the battery negative first, positive last, fully charging the battery and reconnecting positive first, negative last, solved this issue in most cases (my Mustang was one of them). Another issue we had with this gen Mustang was the new gen radio wasn't shutting off when turning off the key, despite a blank display. Turning radio off manually before turning the key off solved most issues. With the Marauder? I dunno, as this is the first time I've read this.
Current Garage:
2005 Mustang GT Premium
2019 Ranger Lariat Super Crewcab 4WD
2015 Triumph Trophy SE
“You've gotta dance like there's nobody watching,
Love like you'll never be hurt,
Sing like there's nobody listening,
And live like it's heaven on earth.”
― William W. Purkey