A second MP could face an investigation into their travel expenses after staff at the regulator raised concerns.
The Independent Parliamentary Standards Authority's (Ipsa) compliance officer Peter Davis launched a formal probe into the DUP's Jim Shannon last week.
A freedom of information request by the Press Association has revealed Mr Davis is also considering allegations about another as-yet-unnamed politician.
Ipsa staff asked Mr Davis to examine travel claims by both individuals more than two months ago, on November 9.
Mr Davis announced a formal inquiry into Strangford MP Mr Shannon - who denies any wrongdoing - last Monday, while the other case is listed as "ongoing".
The watchdog, who operates separately from Ipsa, is obliged to identify politicians when he kicks off an investigation.
But he appears to have been getting round the rule by carrying out in-depth "assessments" and in a number of cases allowing MPs to repay money or make amends in secret.
The compliance officer's arrangements are less transparent than those at the House of Commons, where parliamentary standards commissioner Kathryn Hudson publishes details of complaints that are not upheld or "rectified" with apologies or small repayments.
According to the latest breakdown of assessments released under FOI, Mr Davis declined to launch a formal investigation into a politician after Ipsa officials alleged last April that "staff employed by the MP were engaged in non-parliamentary work during contracted hours".
After carrying out an "assessment", the compliance officer decided there was "no evidence of a scheme breach".
On May 25, Ipsa staff complained that an MP had not provided "sufficient information for a claim for capital expenditure" in the run-up to the general election.
Mr Davis dropped the case without carrying out an investigation after saying he had now received "sufficient information".
In 2014-15, the compliance officer opened 40 cases, but carried out just one formal investigation.