Came out to call for a system making noise. Customer stated that they had come back from vacation yesterday and noticed the AC was running. Customer had to turn off the breakers to shut it off. Had suspicion that it might be the thermostat, but wanted us to look at it to make sure. I checked the system over and did in fact find the nest thermostat sending a signal to the AC even with the thermostat set to off. Pull thermostat off wall and the AC disengaged. Discussed with customer about replacement of thermostat. Customers will install a new one, since they installed the first one. Recommended maintenance on system. Customer will look into having it checked this summer.