Two Knocks

Just the day before yesterday, something happened. My daughter was sleeping in our bedroom, and my mother left the apartment for a couple of minutes (she took the keys with her, as we have an intercom in the entryway). I was pacing the living room, waiting for her to return, as my friend was waiting for me downstairs. Suddenly, there was a knock on the door—literally two knocks (my door is iron, and it sounded like someone gently tapping with a finger). My first thought was that my mother had returned and wasn't ringing so as not to wake my granddaughter, but then a second thought occurred: my mother had the key, and she could come in herself. I walked up to the door and was about to open it, as I wasn't expecting anyone except my mother. I put my hand on the lock and, out of habit, glanced through the peephole. There was no one behind the door, and I couldn't hear footsteps descending. The entryway was quiet—it appeared to be empty...

By the way, I should add that this isn't the first time. Besides that, I sometimes hear my name clearly called, and when I turn around, no one's there. One time, my mother, sitting next to me, also heard a man's voice calling my name. At the time, my father was also home, looking at us in confusion and saying he hadn't called anyone. As older women say, the main thing is not to answer or open the door, otherwise you never know what trouble might be trying to get in...