Atheists in B.C.'s Okanagan have a mystery on their hands after bus advertisements questioning the existence of God mysteriously disappeared this week.

The ads, which were debuted on the broad side of buses earlier this month in Kelowna and Kamloops, read "There is probably no God – now stop worrying and enjoy your life."

The campaign is spearheaded by the Centre for Inquiry Canada, a skeptic group that has sponsored similar campaigns on transit across the country since 2009.

The organization said several BC Transit employees objected to the ads on religious grounds, and two drivers were reassigned after refusing to drive the buses featuring their message.

CFI Vancouver Executive Director Jamie Williams told it appears the two missing Kelowna ads -- which measure two-and-a-half by 11.5 feet each -- were professionally removed.

"Someone with the time and expertise and equipment – solvents – has gone about their work and taken them down," he said. "There was literally no residue left."

BC Transit, the advertising agency and the City of Kelowna have been unable to provide an explanation as to what happened to the giant signs, he added.

While similar CFI ads have been the victim of graffiti in the past, Williams said this latest incident is unprecedented.

"They seem to have disappeared into thin air," he said.

Williams said several transit companies across Canada, including BC Transit, have refused to run the ads, citing policies against provocative or ideological messages.

But the CFI's ads have been approved by the Canadian Standards Association, and many transit providers have since relented – including in Vancouver and Victoria, where the campaign may be heading in the near-future.

A similar ad campaign was rolled out with some controversy on more than 200 buses in Britain in 2009.

Organizers said the Atheist Bus Campaign was intended to place "peaceful and upbeat" messages about atheism on public transportation in response to Christian advertising.