I found this in a related thread:
Quote:
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Originally Posted by Sixto
It might be a bad relay if the bulb is on all the time. The bulb should be off when you first start the engine then come on when you switch on the circuit with the bad bulb, then off when you switch off that circuit. Brakes are an exception. If a brake bulb is bad, even intermittently, the warning light will stay on from when it first senses the bad bulb until you switch off the engine.
Sixto
87 300D
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Same car as me, too!
So Sixtoes says it latches, but only for brake lights. It sounds as if the latch would be cleared when the key goes off, and not re-latch until brake is used again. ... or detected intermittent... So does this mean it can detect even if the light is not used, all by "itself"? Maybe. And that supports the OP's post and my observation of when it comes on, and what fixed the OP's problem.
This makes me want to check my brake lights, before suspecting and yanking the bulb failure module itself.