It does sound like object constancy, which is more of an evocative memory and developmental rather than something that you can do at will. Food and smell memories are another evocative type of memory; they are more powerful than other types of memories.

Winnicott described how children use transitional objects to help tolerate negative affects until this evocative memory is established (around age 2). TOs link the more primitive episodic memory to the developing evocative memory acting as a bridge while it's in development. Maybe you could try that-if you don't want to ask for a transition object, which you might not want to do at this point, you can try a voicemail or business card.

My T recently started ending the sessions differently-where he'll look at me longer and say something kind of nurturing, which reinforces his presence before I leave. Before he started being more supportive like this, when he was more blank slate-ish, I often felt anxious and unsettled.

I think if you go twice a week, there is often a repair-rupture dynamic where you feel frustrated or angry or unsettled or anxious, then reunite the next session for repair.

It is a very real thing-think of how people say "you'll always be a part of me" and "you'll always have a special place in my heart" etc. Introjection is a related concept.

Adding that I wrote this before I read your second reply so don't know if it really applies but leaving it up if you wanted to read.