Overheat issues . . .

Hello,

My 2000 Sienna has been good to me. Have driven her last 30K miles without any issue.

I drive about 30 miles a day.
On my way home from work, noticed that my temp gauge was climbing above the middle. By the time I made it up the hill, it was around 80-85% to the top.

I left the car cool off for about 2 hours. I returned back to the car and saw that the expansion tank for the coolant was empty. I concluded that I must have a small coolant leak somewhere, and that I've finally lost enough coolant to cause me trouble. I filled up the expansion tank to the full line, and drove around for 10 mins.

It seemed fine for about 10 mins, then the temperature gauge started climbing above the middle, and started overheating again.

I turned on the heat inside the cabin to max, and turned on the fans to max (while the temperature gauge was 75%), but nothing but cold air was coming through the vents. Next, I parked the car and opened the hood to expedite the cooling process. At this point, I noticed that the expansion tank was still filled to the full line.

Also, I noticed that the electric fans behind the radiators were not spinning, while the engine was overheating at 75%+.

I scanned the car with an OBD-2 scanner. Interestingly enough, right after the scan completed, the electric fans behind the radiators actually turned on for about a minute, and then turned off. The fans turning on was surprising, because it happened when the car was shut off. Car engine was off, but they key was in the turned position, right before cranking the starter.

What's going on here? Feels like it's a thermostat that's stuck closed ?

Thanks!

David