What stage of development is this?

Toren’s latest development: He has figured out that he fits behind our love seat, so he sneaks behind there and hides:

He never used to want to be so far away from us that he couldn’t see us. Now, he sneaks behind the couch and hides. In fact, he did that the other day and Debi didn’t see him go behind the couch. She was running around the house calling for him and was starting to get quite frantic until he started giggling behind the couch. I wonder what happened, cognitively, for him to think that hiding behind the couch is fun. Thoughts?