Ah ! Thank you. Asked and answered.
It sounds similar to what you get if you use "Remove Parameters".
In fact I just tried it and indeed "Remove Parameters" when I did that to a line creates a line in Non-timestamp Geometry. It gets removed from "Model History" of the model navigator, and goes to Non-timestamp Geometry. Makes sense because now there is no "history" on how that entity was created.
What's interesting is doing "Remove Parameters" on a solid, say an Extrude created in the Modeling tab creates a "Body(nn)" in the Model History and not in Non-timestamp Geometry.
Is there are reason for that????
I'm using NX16 by the way.