Eager to see if the rumors of a hidden Banksy were true, he wasted little time.

On the day he got the keys to the site, Mr. Ellis said, he had plywood that had been screwed to the wall removed, but “it wasn’t one of those eureka moments where we pulled the ply off and oh my god there it was.”

Instead, what he saw underneath was a section of white-painted bricks marked with numbers. “Because it had been painted over, we didn’t actually know what was going to be salvageable, or what was left,” Mr. Ellis said. “We were excited to see the wall hadn’t been removed, but at that precise moment didn’t know what we had.”

Mr. Ellis says he believes that the bricks had been numbered by someone intending to remove and reconstruct the potentially valuable stencil brick by brick. Instead, his company removed a whole section of the wall using a power tool.

“We were building there, so we had to explain to the builders — without telling them what was underneath it — that they had to protect this bit of wall,” Mr. Ellis explained. “If you looked at it, it was just a white bit of wall. You would’ve thought we had gone absolutely mental.”

After a wait of about a week, the wall was carted off to the Fine Art Restoration Company in Carlisle, England, in the rural county of Cumbria. (The first van the company sent was too small for all the bricks.)