A stray file is one which was backed up at some point but is no longer present on the machine.
If a file, which was previously backed up, is renamed, moved to another folder, deleted, or excluded from the backup set, the file that was backed up, and its revisions, become stray.
Consider how the software keeps tracks of files.
The IBU software uses the file path as the identifying quality to track files.
The file path acts as the link between the version of the file which exists on the machine and any versions of the file that exist in the backup data set (in the cloud or Local Vault).
If the indicator (file path) changes or the source (the original file) disappears, the data for that file becomes orphaned or stray.
Using those criteria, an example of the indicator changing would be renaming of the file on the machine or renaming of a parent folder; in both cases the file path for the file we want to back up changes. An example of the source disappearing would be deleting the file from the machine or moving it to a different folder or drive.