This post talks about a new field in the Shiny API of PoGoAPI.net
When taking a photo of Pokemon it is possible for a Pokemon to photobomb the picture and appear in front of the subject. When this happens the Pokemon will then appear in the overworld when you finish with the Pokemon Go camera app. This will then let you catch the Pokemon who photobombed your picture.
Typically if the Pokemon is found in the wild it is also possible to be a shiny when it photobombs your photos. However it is important to note that some Pokemon can only be found shiny when photobombing.