Gandalf’s Possible Inaccuracy in the chapter “The Shadow of the Past”

[Zigûr]

Originally Posted by Alcuin

I don’t think could Ar-Pharazôn even see it – remember that neither Sam nor Frodo could see Galadriel’s ring until Frodo saw Sauron in the Mirror. Sauron’s servants were terrified of the Númenóreans and, despite their servile fear of Sauron, deserted him. Ar-Pharazôn took him hostage back to Númenor: this delighted Sauron: otherwise, he’d never have been able to get there. He had a rather overwhelming personality, and soon cozened Ar-Pharazôn.

I thought of this too, but Isildur had to have been able to see the Ring to have a reason to cut off Sauron's finger to seize it. That being said, if it was invisible, we might assume that it became visible again as his body died.

Originally Posted by Galin

"... there he took up again his great Ring in Barad-dur..."

Some have argued that this need not mean Sauron had left the One behind in Middle-earth

An interesting way of looking at it, especially considering Professor Tolkien's typical specificity of language; it is possible that if he meant "took up" to mean "put on", he would have said "put on".
There's also the simple fact that between bodies he could not "take up" the Ring in any sense, to wear it or wield it.
It's possible that late in his time in Númenor Sauron had had little current use for the Ring (almost as Gandalf tells us Gollum did after long centuries in his cave) and had not actively been "using" it.

It's difficult, in my opinion; perhaps he did not bring it with him to Númenor, but it seems to me that he would have been sore pressed to guarantee its safety if he left it behind, unless perhaps the Nazgûl took it with them when they (presumably) went into hiding after they (deliberately) abandoned Sauron at the coming of the Númenóreans. One assumes he could not have simply left it behind in Barad-dûr; we have no information that Ar-Pharazôn's men went to Mordor, let alone searched Sauron's tower, but if so much of Middle-earth fell under the sway of Númenor at that time it's hard to imagine they would have left their defeated enemy's "capital" alone.