When reviewing ghost pictures, we often receive images that have been taken through glass such as a window panes, door glass, etc. This can make it more difficult to see if there is actually a ghostly apparition within the photo. Many times, people visit museums and take photos of exhibits encased in glass and are surprise to see “ghosts” appearing in the frame.
The most common type of non-ghost we see when shooting pictures through glass is usually a human form of someone who was in the room with the photographer – a mere reflection of either their body or shadow. Often, the photographer’s own reflection of a body part will be mistaken for an entity. It’s also important to know that smudges, dirt and fingerprints on glass will illuminate with light of the camera’s flash and look ethereal and see through. The issue with glass reflections, of course, is that the everything reflected is transparent – see through.
If at all possible, try not to take photographs through glass while ghost investigating. If you have to do so, then work to explain anomalies by first considering natural reflection effects. Try and account for everyone who was in the room (and especially where they were at when the photo was recorded), and do not be afraid to attempt to duplicate the effect to either validate or disprove it.
Source: False Ghost Pictures: Glass Reflection