Occam's Razor: Simplest explanation is usually most likely answer.
KISS: Keep It Simple, Stupid.

"All right, goodnight'.

Major event, perhaps lithium batteries fire. Noxious fumes.

Old school captain taught to turn off Transponder before changing code. Never got back to it.

I was always taught in an in flight emergency to turn off course to prevent conflict with following aircraft.

Easiest? Rotate heading bug (to 90 degrees off course or safe heaven). HEADING select.

Crew overcome by toxic fumes. Incapacitatation.

Flight continues on heading til .....

Fuel exhaustion.

Splash.