Edessa image rediscovered in 525

After the reign of King Abgar V, the city reverted to paganism. To keep the image safe, it was hidden inside a gate wall.

The later legend of the image recounts that because the successors of Abgar reverted to paganism, the bishop placed the miraculous image inside a wall, and setting a burning lamp before the image, he sealed them up behind a tile; that the image was later found again, after a vision, on the very night of the Persian invasion, and that not only had it miraculously reproduced itself on the tile, but the same lamp was still burning before it; further, that the bishop of Edessa used a fire into which oil flowing from the image was poured to destroy the Persians.

https://en.wikipedia.org/wiki/Image_of_Edessa

However, as the Synaxarium reports, Abgar’s grandson, who had returned to the worship of the
idols, wanted to destroy the precious relic. In order to save the Mandylion, the Bishop of Edessa,
having been warned in a dream by an angel, had the rounded hollow secretly bricked up with a tile
and a little burning lamp placed before it.

https://www.shroud.com/pdfs/stlpolveraripaper.pdf

It remained hidden until around 525 when it was rediscovered after a flood damaged the gate wall.

The image itself is said to have resurfaced in 525, during a flood of the Daisan, a tributary stream of the Euphrates that passed by Edessa. This flood is mentioned in the writings of the court historian Procopius of Caesarea. In the course of the reconstruction work, a cloth bearing the facial features of a man was discovered hidden in the wall above one of the gates of Edessa.

https://en.wikipedia.org/wiki/Image_of_Edessa