No updates. I've not seen it since. However, I think I have an explanation. The parking lot at the back of the house is very dark and there is a rather secluded and sunken alley outside the window where the fire escape is. It's quite possible it was the police searching for someone in the area and were checking out the alley.
This is a photo of the alley. It's always dismal, even during the daytime. It receives sunlight only for an hour or so during the summer months. Otherwise, just ambient light. Nothing grows there. Not even the heartiest of weeds.
The window I saw the light coming through is on the left beneath the fire escape. The identical tall, slender window belongs to my neighbour. His kitchen window is on the right-hand wall. (The buildings in the pictures above are directly behind where I was standing when I took this photo.)
