I appreciate your suggestions. They would certainly help me to distinguish whether my enclosure is open or closed and that is what I blamed this incident on. However, upon reflection, I think my problem is bigger than that. This is just another incident when I lost or almost lost Wiggles while allowing him to free roam. So, going forward, Wiggles's activity sessions will be confined to an activity station.
The station I have now is 6 ft tall and sits in the middle of my living room, so I'm unlikely to forget to put the station and Wiggles away. He's on there now. See?
I'm also thinking about getting a small pet tent, like this one, to use an an activity station. Hopefully, between the two of them, Wiggles will get the enrichment that he needs without the danger of getting lost.