How did Frodo get his Mithril Coat back?

[radagastly]

If Gandalf thought Narya would be an indicator that Sauron had regained the One, I see no reason he would have associated the darkness with Frodo's intended destination.

Of course, this report from Faramir pre-dates the Battle of Pelennor and the encounter with the Mouth of Sauron before the Black Gate. Since this is mostly pure speculation anyway, I would speculate that Gandalf would be more likely to make use of his Ring to "enhance" the diversionary impact of the Battle at the Black Gate, as opposed to hiding it before then, and making Sauron aware that one of the Three was right outside his door, tempting him to strike, while at the same time allowing Gandalf the chance to determine whether Sauron had recovered the One (if such actions could be extracted through Narya. I'm not sure how it works, but this is all rather speculative, as has been mentioned.)