A couple of possible reasons. He could be choosing a cooler darker spot to settle for the night. Or, he could have been down on the ground when it got dark (lights off, saw darkness through the window) and as his brain cooled down just got sleepy right where he was.
I had 3 melleri who free ranged in a bedroom. Once in a while the big older female wandered out into the rest of the house but the place I usually found her in the evenings when I got home was in the bathroom behind the toilet. I suspected she wandered in there at some point during the day, it was cool, damp, and shady back there, her brain chilled, so she just stopped moving and napped until I found her. Her other favorite spot was behind the fridge perched on the fan housing, but it was warm there. Then she'd stroll back out for another lap around the house, visit the plant stand in the front window, climb over the couch, down my arm, over my shoes, tongue lash and lecture the "other" cham she saw in the stereo cabinet glass door, go around to the back of the cabinet trying to find the intruder and get hung up in the cables. After being rescued by her personal attendant, she'd get a ride back to her Ficus tree, perimeter patrol completed to her satisfaction.
