airbag light in cluster when no passenger

[snowfox]

2004 BMW 330i ZHP,
The red airbag light in the instrument cluster stays on when there's nobody in the front passenger seat. When an adult sits there it seems to go away, but comes back later. It seemed to start when I placed my toolbag into the passenger seat one day and stayed on since.

I tried disconnecting the battery and reseating all the connections under the passenger seat.
I tried disconnecting the battery, replacing the small black sensor box under the passenger seat with a known working unit pulled from a 2003 BMW 330i.

Neither of these attempts made the airbag malfunction light go away.

I do noticed when turning they car key to POS2 and holding it there with the engine off the airbag light goes off for a brief fraction of a second and then comes back on again.

If anybody knows what this issue is and how to fix it please let me know.

[E39 Newbie]

You probably have a bad occupancy sensor in the seat. You have to pull the seat cushion off, pull off the leather cover, remove and replace the occupancy sensor mat. They are a known weak spot, especially on the E6x series.

A good code scanner can probably confirm this hypothesis.