The auditor for MPs expenses requested the representative for Orkney and Shetland to provide more information on his mortgage repayments – even though he paid rent during the period..
Sir Thomas Legg's review has seen hundreds of MPs requested to re-pay or justify claims.
Liberal Democrat MP Alistair Carmichael said he had been asked to provide mortgage documents for the period from 2004 to 2006.
Mr Carmichael said: "The difference between rent and mortgage is a fairly simple one and I was surprised when I saw that. That kind of jumped out of the page at me.
"It was very noticeable, it did make me think that it was a piece of work that maybe had not been done with all the rigour that it should have.
"If the figures that Thomas Legg has given any MP are, on checking, correct, my view is that you pay and move on."
Mr Carmichael said he would write to Sir Thomas in the next "day or two" and highlight the request for mortgage documents.
MPs have to respond to the letters within 21 days.
