OTTAWA -- For the first time in more than two years, Sen. Mike Duffy returned to Parliament Hill on Monday after the end of his long-running legal odyssey.

Much as he has done since his trial began last year, Duffy stayed mum as he passed through the Senate doors to the Centre Block, adding to the suspense for Tuesday's Senate sitting when he can take his seat in the upper chamber -- and say whatever he wants under the blanket of parliamentary privilege.

Duffy's return to the Hill comes less than two weeks after his marathon trial ended in a sensational acquittal on all 31 fraud, breach of trust and bribery charges he'd been facing.

Duffy was suspended from the Senate over his expense troubles, but the suspension was lifted when Parliament was dissolved last summer in anticipation of the 2015 federal election.

His case marks the first time that a senator has been charged, tried and acquitted, paving the way for questions about whether the Senate jumped the gun on stripping Duffy of his salary before due process had been allowed to run its course

Duffy lost out on more than $250,000 in salary during his suspension -- money that his lawyer has argued Duffy should be paid after his acquittal.

His former Conservative colleague Sen. Linda Frum disagreed Monday.

"The Senate made a decision about his salary; it was part of a sanctions process against Sen. Duffy, and that sanctions process took place, and that's the end of that story," said Frum, who was one of those who voted in support of the suspension.

Nonetheless, he has "every right to be here," she said, adding, "I hope he'll devote himself to his job with a kind of integrity that perhaps was missing in the past."

Duffy getting his salary or pension back would require a motion to the Senate, which could be moved by any senator, himself included. The Senate as a whole would have to vote on the motion, just as it did on the vote to suspend.

The Senate has already allowed Sen. Pamela Wallin, also suspended over her own expense controversy, to start accruing time needed to qualify for a pension -- a clock that remains stopped for Duffy and Sen. Patrick Brazeau until the Senate decides otherwise.

NDP MP Charlie Angus told reporters he doesn't see a reason to give Duffy back the salary he lost.

"What we're seeing is a real disconnect between the Ottawa political bubble and ordinary Canadians who work hard and play by the rules," Angus said.

"Now we're supposed to be all feeling bad for a senator who's going to go back and demand all his back pay for all the time that he was in court?"

In dismissing the charges, Ontario Court Justice Charles Vaillancourt said while Duffy's expenses for travel, housing and contracts may raise eyebrows, they did not amount to criminal behaviour. Nor did the Crown prove beyond a reasonable doubt that Duffy was guilty.

Instead, Vaillancourt set his gaze on the Senate's vague spending rules and Stephen Harper's Prime Minister's Office, delivering a scathing critique of how it tried to manage Duffy's politically problematic expense claims.

The judgment showed the Conservative government deserved "some blame" for what happened, in particular the "people who were in charge" at the time, Conservative leadership candidate Maxime Bernier said Monday.

Many of the operatives in the PMO are now gone, while two of the top senators involved in the Duffy affair -- former senators Marjory LeBreton and Irving Gerstein -- have since retired. The Senate has also been through an auditor general's review and has amended its spending rules.

Indeed, the Parliament Hill where Duffy arrived Monday is a different place from the one he left.

The Senate changed a little more on Monday as another senator chose to sit as an independent without an affiliation to any caucus.

Sen. Grant Mitchell said he was leaving the Liberal Senate caucus to foster what he sees as the best way to "address the pressure for change facing the Senate."

Mitchell's move brings to 22 the number of independents in the Senate, including Duffy and Wallin.