New oil sensor or just low oil?

My oil light came on while driving home this evening. This has happened before. Each time it comes on, it stays on for a minute or less (doesn't blink) and goes out. The oil pressure does not go down but remains at 3--as it should be when the car is in motion--when the light comes on. When I check the oil level, the oil reads either just below the midpoint or .25" above the red LOW mark. I should point out that I check it a few minutes after I pull to a stop, so the reading should be a little higher when all the oil has settled down. Usually I check the level again in the morning when the engine is cold and has settled down. The level then reads either at the midpoint or a little above it.

How low does the oil level go on the dipstick before the sensor comes on? I drove my car home for another 10 miles in stop-and-go traffic when my oil light came on because from experience, my oil level usually isn't on or below the red LOW mark when the oil light comes on (and doesn't stay on). And the oil pressure was high. Am I fine? Is this a sensor thing or a low oil issue? Car is a '90 190E. Thanks.