"I think," Hermione paused to make it look like she was speculating. "I think Harry told me that new security measures were being put in place, specially for Floo travel and communication. While your office hearth should be the only one open for travel, it appears to have been switched with the one in the common room. Perhaps as a Floo portal, it was rerouted and unlocked accidentally in Harry’s attempts to tighten security for your office. It’s the one Harry and I use to chat."