The structure disappeared several days after it was first spotted near the Petrodava Dacian Fortress, a local archaeological landmark in the northern Neamt country.
Shortly afterwards, a new monolith was spotted at the top of a mountain trail in southern California before it also vanished.
Then, over the first weekend in December, people stumbled across a tall shiny monolith on a beach on the Isle of Wight. The pillar was spotted on Compton Beach on the west side of the island.
However, when asked about the Isle of Wight structure, it said: “The monolith is out of my control at this point. Godspeed to all the aliens working hard around the globe to propagate the myth.”
A monolith on a red-rock desert in an undisclosed location in San Juan County southeastern Utah. File/AP
The structures have sparked comparisons with the movie 2001: A Space Odyssey, where an alien monolith is a recurring symbol that appears to play a role in the development of human evolution.
Bret Hutchings, a helicopter pilot who first spotted the Utah monolith, told local news station KSLTV he thought the structures could be from “a new wave artist” or someone who was a fan of the Stanley Kubrick film.
No one has claimed responsibility for the European monoliths.
Speaking about the Romanian monolith, the Piatra Neamt mayor Andrei Carabeleawrote on Facebook: “There is no reason to panic for those who think there is still life in the universe.”
A monolith stands on a Stadium Park hillside in Atascadero, Calif. File/AP
“My guess is that some alien, cheeky and terrible teenagers left home with their parents' UFO and started planting metal monoliths around the world,” the mayor said. “First in Utah and then at Piatra Neamt. I am honoured that they chose our city.”
Speculation has also followed the disappearance of the US and Romanian monoliths.
A Colorado photographer told KSTU-TV that he saw four men come to the remote Utah site one night and push over the hollow, stainless steel object, saying “leave no trace” as they walked away.
Two extreme-sports athletes have claimed they were part of a group that tore down the hollow metal structure because they were worried about the damage the droves of visitors were causing to the relatively untouched spot.
Officials said the visitors flattened plants with their cars and left behind human waste.
Meanwhile, authorities in the Californian city of Atascadero said its monolith had been torn down by a group of young men that traveled five hours in the middle of the night to remove it.
“We are upset that these young men felt the need to drive five hours to come into our community and vandalise the monolith,” Heather Moreno, the local mayor, said. “The monolith was something unique and fun in an otherwise stressful time.”