When my art classes are going well, the students almost vanish into their work.
It happens to me also. I get so engrossed in my artwork that I forget myself for minutes or hours sometimes. When I finally come around, I am relaxed but surprised by how much time has passed.
So, I am frequently amused when it happens to my students. The kids descend into their work, and I wait happily to see which one of them will reemerge first, break the quiet, and start her classmates buzzing again.
I love the term for this by the way: parallel play. "A form of play where children play adjacent to each other, but do not try to influence one another's behavior."