The firms received a letter from Cherrytree - a cosmetics company that seems not to exist - asking for credentials and claiming it was shortlisting agencies.
A north London mailing address and the name Sarah Moustaki can be linked to a rival agency, Blazinstar Experiential. Errica Moustaki, who lives at the address, lists herself as a director of Blazinstar Experiential on the Linkedin business networking site. The claim on Linkedin has since been removed.
Blazinstar, co-founder Shaz Smilansky said she could not comment, but was "investigating". Smilansky also denied "100%" Moustaki is a director.
Ian Irving, sales and marketing director of Sledge, received a letter from Cherrytree then told Event about it after having undertaken his own investigation.
"I realised something was not right, so I did a little digging," said Irving. "If we had sat down and put it [the credentials, plus some ideas for the brief] together, it would have cost at least a grand."
According to RPM managing partner Hugh Robertson, who also received the letter, it could be damaging for the industry.
"There are three [issues]: the time you invest; the confidential information that you send; and the damage to the industry as a whole," he said.
Did you receive the mystery letter? Let us know. and comment below.