Hey Ian- I know what you mean. H has done it on many occasions.
My thoughts are this. At least for me I'm afraid to relax and whatever (I kept busy too!) because what if the next time he doesn't stay longer. What if he decides he doesn't want to get back together and I get hurt again? What if I get comfortable and he pulls back and leaves me hanging again? What if he starts hanging out here more and we slip back into old habits and things go sour?
So know if he hangs around, I invite him to help me wash the dishes, or to watch the news with me, or to run an errand with me.
I become pro-active instead of reactive. I think that is why H and I are where we are right now.
