The researchers built mathematical models to account for these observations, which suggest CFC-11 emissions have actually been increasing by around 25% each year since 2012, despite virtually no CFC-11 production being reported to the relevant authorities during this time.
"I've been making these measurements for more than 30 years, and this is the most surprising thing I've seen", the paper's lead author, Stephen Montzka, told The Washington Post's Chris Mooney. "I was astounded by it really". The discovery is likely to encourage the worldwide investigation of a mysterious source.
Under the Montreal Protocol, the world agreed to begin phasing out CFC-11, ending its production altogether by 2010.
"It's worrisome that someone's cheating", he said.
But for now, the scientists don't know exactly who, or where, that person would be. Some scientists speculate that the substance is likely being produced in East Asia.
"It really looks like somebody is making it new" in violation of worldwide law, says Montzka. Their results were published in the journal Nature. The toxic CFC-11 is completely banned across the globe.
At first, the researchers hypothesised that the sudden hike in CFC-11 might be due to the destruction of old buildings containing CFC-11 refrigerants. It is thought that about 13,000 tonnes a year has been released since 2013. Together, this analysis suggested the emissions are coming from east Asia.
Nations in the Montreal Protocol have reported close to zero CFC-11 emissions since 2006. That loss of ozone, in turn, weakens our protection from UV radiation at the Earth's surface.
The find was made after NOAA witnessed a spike in trichlorofluoromethane or CFC-11, a type of chlorofluorocarbon and the second-most abundant gas in the atmosphere contributing to the depletion of ozone layer.
The finding that the destruction of ozone was creating a large "hole" over the Antarctic led to the signing of the Montreal Protocol in 1987.
Nearly no CFC-11 has been been produced since 2006 - or so we thought.
Watson suggested that aircraft flights might be necessary to better identify the source of the emissions.
"We're raising a flag to the global community to say, 'This is what's going on, and it is taking us away from timely recovery of the ozone layer, '" said Montzka. As expected, CFC-11 levels have been declining since the chemical was banned and production phased out.
"This treaty can not afford not to follow its tradition and keep its compliance record", he said. "That's a tough group of people".
Despite the increase in CFC-11 emissions, its concentration in the atmosphere continues to decrease, but only about half as fast as the decline observed a few years ago, and at a substantially slower rate than expected.
But in 2012 scientists noted that the rate of decline had slowed by 50 per cent, according to the new study. But if the problem is allowed to persist, it could jeopardize ozone layer recovery and worsen climate change. Though concentrations of CFC11 in the atmosphere are still declining, they're declining more slowly than they would if there were no new sources, Montzka said.