(CNN)A mystery SOS sign in a remote part of Australia has confused police after a search found no evidence of “recent human activity” in the area.

The large sign, made of rocks, was spotted recently by a pilot flying over Swift Bay in Kimberley, Western Australia’s northernmost region. A police ground search followed.

The emergency signal raised concerns that people may be missing or stranded, but after an inconclusive investigation, authorities are now appealing to the public for information.

“Police have conducted a ground search of the area but did not locate any persons or any indication of recent human activity in the vicinity,” Western Australia Police said in a statement Thursday. “Police are trying to establish if there are any missing persons in the area who require assistance,”

It’s also possible that the distress sign was created months ago and went unseen, or that the sign is simply a prank.

SOS signs can be crucial to rescue operations. In 2014 a similar sign on a sandbar led to five snorkelers being successfully found near Australia’s northeastern coast.