The RCMP says Duffy spent 34 nights in Charlottetown hotels between 2009 and 2012 instead of staying at his declared primary residence in Cavendish.
Last week, lead investigator Cpl. Greg Horton asked for and received a search warrant for hotel records.
Duffy, along with fellow former Conservatives Patrick Brazeau and Pamela Wallin, was suspended from the Senate in early November over expense claims.
Liberal Mac Harb resigned from the Senate in August after paying back $231,000 for ineligible housing and travel expenses.
Harb and Brazeau face criminal charges of fraud and breach of trust, while Duffy and Wallin have not been charged.