Thirty firefighters battled five hours Saturday and Sunday against a fire that passersby would not have noticed.

The first truck arrived at condos at Teton Pines soon after 10 p.m. Battalion Chief Mike Moyer said firefighters saw “nothing obvious” when they pulled up at the building, near the golf course clubhouse. When they knocked on a first-floor condo door the people inside “were unaware of any issues.”

The only fire they had seen, the visiting renters said, was in their fireplace.

But it ended up being a case of a fire alarm that turned out to be a real fire, Moyer said.

In this case, firefighters walking around the building spotted “a glow,” Moyer said, and soon found their fire: After apparently starting in the fireplace chaise, it had spread under the building’s cold roof and had been burning for an unknown length of time.

“By the time we got there it had been burning for a little while before penetrating into the building and setting off the fire alarm” in an empty second-floor unit, Moyer said.

Finding the fire didn’t make it easy to extinguish it. Moyer said it was protected by the roof.

“We applied a lot of water from the outside,” he said, “and ended up using our 100-foot aerial to get on top and cut some ventilation holes in the top, where the fire was embedded.”

The fire was officially dead about 3:30 a.m. No one was injured, though renters in several units were moved to new vacation lodgings. Moyer said the building would need “significant repair.”

The fire alarm was a big factor, Moyer said, alerting help long before the need would have been noticed otherwise.

“We would have lost quite a bit more of the building” without the alarm, he said.

“A lot of fire alarms are false alarms, but we treat them all like they are the real thing,” he said. “And sometimes they are.”