Just picked up a 1999 S430. High miler, cheap cool daily

anyway, it has a battery drain issue. The battery is certainly knackered but potentially coincidentally there is a problem where I turn the engine off, I remove the key and open the door - a warning comes up on the dash screen saying “Atom light remove key”. I presume this means the car thinks the key is still present and therefore leaving the automatic headlights on (which is happening). the key is certainly removed so I just thought I’d check if anyone has come across this issue before?

thanks