[jimminy]

"They think in their head, don't work through the problem at all."

Those are independent clauses. Just because the individual is not talking out loud, should not be indicative that they aren't working through the problem. Personally, I am a very visual thinker and layout visual models in my head for problems with high (3-5d) dimensionality that would take longer to draw clearly in 2D space while explaining all the traversals I'm mentally making. I would then reduce a solution, and explain that, or I apologize for zoning out.

It became obvious to my office mates when I was working through and reducing a problem space, because I would "hang" on the thought. Sometimes I would be down, stuck in thought, for 15-30 seconds and according to them they could tell because my face loses emotion and my eyes flutter about.