2000 Malibu Radiator Fan Diagnosis?

2000 Malibu. Lately, have not noticed the radiator fans coming on, and it occasionally boils out the expansion tank if I sit in traffic after getting off the (70mph) freeway.

I did notice that if I turn on the a/c, both fans come on as they should. But then, after I turn off the a/c, even if I have it on for only a second or two, both fans keep running until I turn off the car.

I'm thinking relay or temp sensor. Thinking relay more because the fans keep running after the a/c operation kicks them on. Is there more than one relay? As in, one to control a single fan, and then another one for a/c-related 2-fan operation? Are these relays in the under-hood relay block?

Any suggestions greatly appreciated.; Thanks; Carl