I turned around and didn't see anything, so I went back to what I was doing.
Soon I heard another tapping, and I turned around and saw a tiny little bird on the outside ledge looking in. Not at me, but into the room. He kept coming back, so I took some pictures.
In this first one, you can see him flying:
In these two he is down near the bottom left corner of the window. You can just see his head:
One other time before I could get down and take the picture, he was perched on the divider of the two windows looking in sideways.
As I kept looking in, he retreated into the bushes, and didn't come back.
I told Jennifer about it, but didn't think anything else about it. Until today, when she called me to tell me the bird was back.
Apparently it's taunting Pom. Jennifer can see the window from her desk because our desks face each other. She said the bird kept tapping until Pom came in to look at it. Then of course, she wanted out to go chase the birds.