At least 46 senators have been recorded as paying back expenses since the fall of 2010, according to a Globe and Mail analysis of Senate financial reports.
The Senate does not include the reasons for returned expenses – and a spokeswoman would not say why the information isn't public.
She did say the most common explanation is accounting for prepaid airline tickets that may have been used or refunded in a different reporting period.
Or they may be claims that were rejected.
The Globe and Mail reviewed quarterly financial reports for the period between the fall of 2010 and the end of November, 2012.
The Senate has faced calls for greater public accountability as a small but increasing number of members' spending of public funds has been called into question.
The Globe review shows nearly half of the Senate has at one time or another listed an expense category as a negative number, meaning money was returned.
The negative entries include two for travel from Conservative Senator Pamela Wallin – one for $1,237.96 and another for $14.41 – and one for $4,200 from Conservative Senator Mike Duffy. Both are facing external audits related to their expenses. Neither would answer questions about the negative entries.
However, the number given as the total for that quarter in each category is the total of approved expenses and amounts returned, meaning the negative entries – most of which are less than $1,000 – do not show exactly how much was repaid. Also, if money was returned but the amount was less than expenses claimed in that quarter, there would be no indication at all in the records that a senator had paid money back.
A spokesperson for the Senate says there are many reasons a member might declare a negative expense.
"The main reason for negative amounts is credits from airline companies," Annie Joannette said, pointing out that sometimes a ticket was bought in one quarter and cancelled or refunded in a subsequent quarter.
Ms. Joannette said the negative expenses could also indicate the senator claimed something Senate financial officials later disallowed. (The totals of the expense claims senators submit during a quarter make up the quarterly report.)
"Each claim is reviewed line by line by two separate financial officials," she said in an e-mail. "If an expense is not allowed under the rules and policies, or if the paperwork is incomplete, the claim is refused or challenged."
Questions over the Senate continue to dominate the political debate on Parliament Hill, with the NDP saying that a Senate committee's ongoing investigations of sitting senators are not transparent. The New Democrats have no senators.
Various reviews are taking place to determine the appropriateness of senators' spending, as well as whether they meet their constitutional duty to reside in the province they represent.
"All senators conform to the residency requirements. That is the basis on which they are appointed to the Senate and those requirements have been clear for 150 years," Prime Minister Stephen Harper said on Wednesday.
The comment raised eyebrows from the opposition, which said the Prime Minister appeared to be prejudging the results of the residency investigations.
"The Prime Minister's saying everything's fine, move along, just like he said about Ms. Wallin," said NDP MP Charlie Angus. "He told us he examined her travel claims and everything was fine. Now we find out she's had to pay money back."
The government has yet to comment on or refute a CTV report this week that Ms. Wallin recently repaid a "substantial" amount of money before meeting with auditors to discuss her expenses. Ms. Wallin had previously told CTV that "no offer of repayment was made or was asked for."
Government Senate Leader Marjory LeBreton and Liberal Opposition Leader James Cowan asked members of the Senate committee on internal economy earlier this month to interview senators who did not provide adequate documentation showing where they live and pay taxes. The interviews were scheduled for this week.
"We've set in place a process, and I'm confident the process will work, and we'll make it all public," Ms. LeBreton said on Wednesday.
Conservative Senator Dennis Patterson, who represents Nunavut and is reported to own residences in both Iqaluit and Vancouver, has so far declined to say if he was called in for an interview. Speaking outside the Senate chamber on Wednesday, he did not respond to questions about where he pays taxes and has a health card.
"I respect the rules and procedures of the tenets of the Senate. I think I'm in full compliance with them," Mr. Patterson said. "I'm co-operating fully."
Liberal Senator Serge Joyal said he believes questions of residency and improper expense claims are limited to a small number of senators. "I don't think that the majority of senators are, you know, under a cloud of suspicion," he said.
Asked if the police may need to be called if evidence of wrongdoing is found, Mr. Joyal said, "the law is the law."
"First of all it's up to the Senate to do the first evaluation of the situation, and when they come to a conclusion, then they have to transfer the decisions to the proper authorities," he said.
As for Senate expenses posted online, the biggest negative expense listed to date is a $11,145.90 entry by Conservative Senator Elizabeth Marshall, a former auditor-general of Newfoundland and Labrador who leads a sub-committee reviewing the expenses of Independent Senator Patrick Brazeau and Liberal Senator Mac Harb.
The negative expense was recorded between Dec. 1, 2010, and Feb. 28, 2011. Ms. Marshall, who was appointed in January, 2010, said she had bought a large number of pre-paid plane tickets and claimed them all at once. She later decided to claim each one when she used it, which led to some revisions to her reporting.
Senate communications would not explain why returned expenses are not reported separately.
"The quarterly financial reports are the information available," the spokesperson for Senate communications said.
With data analysis by Stuart A. Thompson in Toronto