My 5-year-old has set up a selection of large rocks on the railing of my back porch, which my kitchen faces.
In the kitchen, there is a small TV with a red status LED, which due to the window construction reflects twice.
As I naively walked into my kitchen, the only person in my house awake in the early morning, the rocks and lights lined up in an alarming way. I got quite a start.
(I know the picture isn’t great quality … I think the phone camera didn’t want to focus on reflections. Also I zoomed in to hide my kitchen clutter.)
edit: Clarifications.
The reaction was definitely instinctual and guttural. It looked like it was about to pounce and it had clear glowing red eyes that appeared to be focused on me and moved as I did. I couldn’t say what size I interpreted it to be, but it was momentarily quite startling.