Percy had asked Cass to go upstairs as he spoke to Harry. It wasn't that he didn't trust Cass. He trusted him implicitly, but the less he knew, the better for everyone involved.
The knock on the door was followed by Plato's barking. Percy wasted no time, went to the door and opened it to let Harry in. He really didn't want anyone to see the man standing in front of his house, not when they were about to go after Umbridge.
"Harry, good to see you," he said formally. "Please, come in. I've set up in the living room," he added as he walked to the room in question.