The source said the detained was the company's chief of economic security.
In early September, a Moscow court sanctioned the arrest of a vice president and a deputy security chief at the Euroset company.
Vice President Boris Levin and Andrei Yermilov were accused of abduction and extortion, among other charges.
According to media reports, investigators said Levin and Yermilov abducted Euroset shipping agent Andrei Vlaskin in 2003.
The Kommersant daily earlier quoted a Euroset source as saying that Vlaskin had stolen large batches of mobile phones, which he then allegedly sold in an outlet he had opened for the purpose.
Vlaskin's alleged crimes were exposed as a result of internal investigation, reports say, following which the shipping agent fled. He was placed on the police wanted list, but the company's security service were the first to find him and "persuaded" him to repay the money they said he had stolen from Euroset, the source said.