I put overheat in quotation marks because I'm not entirely sure if it is overheating.

This is the situation:

I was on a long, 1000 km drive and 3/4 of the way through, when I stopped to get gas, I had tons of coolant bubbling out of the resevoir (temp gauge in interior was only reading slightly above halfway), I let her cool down for about an hour then took the thermostat out (thinking it might have siezed closed)topped up the coolant, then set back on my way. A couple hours later I pulled over again to check the coolant level, it was slightly low (had to let the new stuff recirculate) but when I shut off the car, the coolant violently bubbled out of the resevoir! This continued to occur everytime I stopped. Coolant level would be low , and I would shut off the car, and it would boil over. Then it occured to me, I have NEVER, since I bought the car, heard the fan on once the car was shut off. Even in quite hot weather, with city driving and A/C on, the fan has never come on when the car has been off. It comes on when the car is running though. I'm thinking that since the only thing to cool the car once off is the fan, and since it isn't coming on, that something is wrong with the coolant temperature sensor, a relay, or the fan itself. Does anyone have an idea how to find the exact problem? Would there be a code thrown into the OBD-II system without a check engine light coming on? This is a very weird problem and the retards at the local dealership didn't give me any help and tried to tell me it was the thermostat (sure guys, that makes sense since the thermostat is out!)