Dirty or loose contacts or bulb nearing the end of it's life. Current for both brake lights goes through the "bulb out" relay. It works when the current drawn by both lamps is equal. It's a reed switch held between contacts by a magnetic field created on either side of the "reed". If the current is zero on one side as when a lamp is burned out or loose, the reed is pulled to a contact and your dash light goes on. If you replace a lamp with an incorrect type with a different resistance, the current is always unbalanced and the dash light will go on, even if both brake lights work.

If you have a bulb about to fail due to a compromised filament, the current can be affected and the dash light might be triggered.

My wife's 940 is doing this now and it's either a loose socket or other intermittent. If I smack the brake light housing it comes on and the dash light goes out. Next on my list.

Erwin