Gf's mom

I was in my early 20’s and dating a girl a couple of years younger. One night we were saying a lengthy good night on the back porch, with her against the wall and me facing her. We were right outside the bathroom window, which was open a few inches. We weren’t talking, just lots of kissing and petting going on, so were very quiet. Then the bathroom light came on and I was looking through the open window at the bathroom mirror. Reflected in the mirror, I saw my girl’s mom walk in and start undressing. She took off her top then her bra. I remember thinking that, although in her 50’s, she had a great pair of boobs, firm and not hanging much. Then she saw that the window was open, closed it and my view was gone. I don’t know if she realized that I had seen her. My girl didn’t see it but I told her and we had a good chuckle about it.