I know last month when H and I were doing our "calendar" thing, he spent most of 'his' nights at our house hanging out with the kids b/c he had nowhere to go with them. It was odd, I didn't want to feel 'underfoot', etc.
I know yours is a different situation, but I can empathize somewhat.
Does she talk to you while she is there? I know you said it isn't tense or anything, but I just wondered if you guys interacted or if you just headed to the garage once she came over...
I forget, I thought she had a place? Could she not take the kids over there? Or take them out sometimes? Or do you think she missed home (I don't necessarily mean you), being at her 'home' with the kids?