So my car recently overheated. I pulled over immediately and had it towed. I took it to the dealership and it ended up being that the expansion tank had "ruptured" so i had it replaced. now the cars Low Coolant light comes on and off. At first I thought maybe they didnt bleed the system properly so id let the air out myself. and that seemed to help a little, however now the light comes on whenever im sitting still or driving slowly. Temp gauge reads normal, no visible leaks, and the dealer told me they pressure tested the coolant system and head gasket when they fixed it and assured me everything was fine. i brought it from a used car dealer not a bmw dealer. I brought the car with 85k miles on it and it now has 91k. Any idea whats going on?