engine fan runs after I turn the car off

Anonymous says "car runs for about 30-60 seconds after car is off" but I'm guessing Anon means the *fan* is running after the engine is turned off. I experienced this with my Altima (1996 which I have had since new). Thermostat was replaced and ever since the fan will run for about a minute after car is turned off, even after a very short distance when engine is definitely not hot. The fan NEVER did this before and like I said, I bought the car new. I called the mechanic and he told me to bring it back in to check it out, since he first said the fan continuing to run when the engine and outside temp are not hot should not be happening. After going over everything he said things appear to be working normally. I trust this mechanic -- he's honest and has "gray hair" (as in been at it a long time). But the fan running like it does now is very unnverving. If this is normal -- fuse relays working correctly, as you say - why did this NEVER happen before? Were the fuse relays not working correctly for the first 14.5 years I owned the car? Would a newer thermostat be designed to work differently?