Thanks for the support, everyone. Yes I could benefit from a CoolantSnitch. Yes it was the aluminum bodied MTC thermostat; while waiting for the tow truck and trying to bleed the system, the heater circuit on the MTC thermostat also blew with a "pop" and small puff of smoke.
I felt guilty enough about it that I just changed the oil ahead of schedule, using a BMW approved Total Quartz oil, whereas before I was running Penzoil Ultra synthetic., which is supposed to be good stuff, but is not BMW LL approved. Not that the type of oil has anything to do with the cooling system, but it took away a little of my guilt.
Lowering my car's resale value one track day at a time...