BOSTON — Earlier this week, something strange happened to this city’s tallest building. High in the air, on one side of the glimmering monolith known to most at the former John Hancock building, materialized a softly rendered figure, standing on some kind of platform.

It was a two-dimensional apparition, a man clad in shorts, looking away from the viewer, reaching from the 44th to the 50th floors of a western-facing side of the tower. The figure was so big and so high up that it was visible from various points in the city.

There had been no public forewarning, and the installation took Bostonians by surprise. They took to social media to ask what it was, they watched it from the online camera of an observatory nearby.

Have you seen the strange figure that showed up on the Hancock today? What do you think? //t.co/ei6k3SaYDl //t.co/uyV54ILIIH — WBUR (@WBUR) 24 Sep 15

“Whatever is going on on the #hancocktower right now” is really weird, tweeted another user, adding an expletive for emphasis.

Then the answer emerged: It was an untitled installation by the secretive artist JR, and the third and final piece in a series of public works at the glassy Boston tower now known by its address, 200 Clarendon.

“We didn’t want anyone to know it was going to happen,” said Pedro H. Alonzo, the Cambridge-based curator who commissioned the piece and curated the series at the building. “That’s the way JR likes to work. He likes to do these things quietly and then people realize wait, we’re surrounded by art.”

Suspended 500 feet above the ground, it is the highest work JR has ever installed, according to Mr. Alonzo. It is 150 feet tall and 86 feet wide, printed on perforated vinyl, and was installed by a local company using the tower’s built-in window-washing rig.

The work seems intentionally vague, with little to distinguish the man or to specify his surroundings, though a ladder appeared on the platform as the piece was completed on Friday, making it look like a dock floating on water. “Summer’s over whether we like it or not,” said Mr. Alonzo, “and there’s this guy standing there in his shorts or his bathing suit.”

The installation is the latest addition to this city’s growing public art scene. This summer, residents and visitors have flocked to installations like the enormous aerial sculpture by Janet Echelman, suspended in the city’s Financial District, and ring-shaped, light-up swings by Höweler and Yoon Architecture at a park called the Lawn on D in South Boston.