Who Was Knocking?

An Experience at Work I work in an administrative position. I'll omit the details, but our office closes in the evening and we set our phones to an answering machine. Basically, after the office is closed, we don't attend to visitors. However, there are times when we have urgent visitors, and in those cases, they knock on the office door and call out. This experience happened one day when I was working overtime. Several other staff members and my supervisor were also working late, and we were quietly doing our jobs in the silent office. Suddenly, there was a "knock knock" on the office door. Usually, if it's a visitor, they'll say "Excuse me" or something after knocking, but I heard nothing. As everyone in the office exchanged glances and tilted their heads, I, being closest to the door, went to open it. When I opened the door and looked down the hallway, there was no one there. I had definitely heard a knock. "There's no one here," I muttered, and everyone in the office stirred. Everyone had definitely heard the knock, but there was no one in the hallway. "For the past few days, this phenomenon of hearing knocking sounds when no one is there has been happening," a colleague working overtime with me told me. Who on earth was knocking? It's an experience that still puzzles me today.