Sauron's spirit was
in the Ring. Or bound to the Ring. So maybe it's not so unlikely that he brought the Ring back. But let's think about the ramifications of this for a moment:
Sauron lost the Ring in the Second Age. He was not able to carry it back to Mordor. If he HAD done this before, than this implies that he lost more power when Isildur cut the Ring off of his finger than when Numenor drowned. But Sauron's physical form perished in the drowning of Numenor, and did not perish
when Isildur cut the Ring. (I. e., it was merely a flesh wound.
) But Sauron's physical form desolved shortly after, which indicates that he could not be separated from the Ring. Which means that he must have had the Ring with him in Numenor.
Then why couldn't Sauron take the Ring back after Isildur took it? In Numenor, his body either perished by drowning or
if the Ring was separated from him. If the Ring was separated from him and he managed to retrieve it, why could he not do so later? He grew just as powerful during the War of the Ring, yet he couldn't even find it!
So his body must have drowned, and his spirit took the Ring back to Mordor. I won't dwell on how. So the only difference between the two perishings of his physical body was that in one, the Ring was taken from him. This must have diminished his power like the other did not, which means that... uh... I've taken a long time to get to an obvious point?
But wait! That means that it was the act of being separated from the Ring that crushed Sauron. Is this why he couldn't take physical form again, unless he grew so much more powerful that the absence of the Ring didn't matter (as much)? Hmm. The first perishment (is that a word?) of his body lost him the ability to appear in a fair form. The second lost him the ability to appear at all. I'm sensing a connection with Gandalf and Saruman, but my brain is too fried to pursue it.