I'm pretty sure if my therapist saw me sitting in the parking lot at a different workplace of his I had no normal reason to be at, he'd be concerned. In his shoes, I certainly would be. Showing up at someone's workplace to see them or watch them without permission is a really common behavior for stalkers.

I in no way mean this as an accusation or slam against the OP. The urge to be closer/catch a glimpse/drive by [or sitting in their parking lot at night *ahem*] for clients feeling attachment pain is completely understandable, and I think it is admirable that the OP is dealing with it, and is looking at it from their therapist's perspective as well.