It's not just outside reflections that can cause us to see an image in a window. We also need to know what is in the room. Is the suspicious image a chair, door or lamp?
What we are talking about is something called matrixing. Matrixing is seeing familiar things in random patterns and shapes.
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As children we would look up at a cloud and see a bunny, a bird or maybe Uncle Fred wearing a cowboy hat. That is the perfect example of matrixing and is a plague for inexperienced ghost hunters.
If a suspicious image is captured in a window, go back to the site at the same time of day and under similar weather conditions. Take several pictures of the same area, at the same and different angles. Take into consideration all possible interior/exterior causes. If you cannot go back to the building, it is best to dismiss the image.
We were at a reputedly haunted restaurant on an investigation a couple of years ago. One of the common, recurring paranormal occurrences was the apparition of a woman's face in an upstairs window. Naturally, during the investigation we took many photos and video of the window to see if we could capture the image.