Speaking with her co-stars Fitzy and Wippa, McGilvray said she noticed a "smell that wasn't right" as soon as she returned home after the open house.
"I was sitting with Mum in the living room and I just couldn't get my head around a smell that wasn't right," she said.
"I thought, 'Are those flowers off? No, they're brand new — what is that?' There was something a bit weird.
"I went to the bathroom and there's footprints on the floor. But they didn't just walk into the bathroom.
"They've taken their liberties with it … and used the facilities, shall we say."
McGilvray, who said she had spent "hours scrubbing" and "sprucing the joint" before the open house, said the mystery toilet user had tried — and failed — to keep things clean.
"To their credit they did pull the chain … but left a mess," she said.
The trio laughed about the situation, and McGilvray confirmed the mess in question was "not a tinkle".
She said the agent had confirmed "quite a few people" had visited the house during the inspection, and that the bathroom user had put the couple in an awkward and embarrassing situation.
"Anyone who came through afterwards would have thought it was us and that we don't know how to clean up after ourselves," she said, before her co-stars jokingly asked whether the notorious Queensland "poo jogger" Andrew Douglas Macintosh had been present on the property for the inspection.
The NSW couple, whose first child, William, was born last year, plan to move to Sydney's North Shore to be closer to family after their house is sold.
The three-bedroom, one-bathroom home on a 135sq m block, boasts harbour views.