I was thinking 2B,
If you really want to know if the PO box isn't valid anymore...offer to fill out a "Change of address" form from the Post office for the PO box that way ANYTHING addressed to the PO box will come to the house. If he freaks out about that then one of 2 things, 1) He still has the PO box in his name or 2) There were things coming to that PO box that he doesn't want "redirected" to the house. I think the reaction will tell much. If he is nonchalant about it (whether he says yes or no), than there's nothing to it except he wanted to feel some "separation" from you (something MLCers love. My W would find all kind of ways to try and "feel" separate from me ,like getting separate phone plans even though it cost a lot more). If he gets all upset than there is more to it and he was getting mail he didn't want you to know about.

Not that it matters except for the fact that the bill has to get paid. What he got if he got anything at all makes zero difference. Kind of like snooping to see if there is OP, never a good idea because all knowing can do is make things worse. Hang in there 2B and let us know how things are going!