Our rat terrier did this to my hubby once when we took him to doggie daycare not long after we got him. He also *tried* one other time when we were in our first obedience class. Both times, there were other dogs around and we hadn't had him long. So we're pretty sure this was his way of marking his "territory."
He's never tried it since, now that he's more clear about his place in the pack. Have you had Charlie long? If not, maybe he's just unclear about the order of things. If so, perhaps a quick refresher in NILIF would be in order. I really wouldn't worry too much about it, though, unless it starts to become a habit or other behavioral problems appear.
Don't you just sometimes wish you could ask them what in the heck is going on in those brains of theirs?