Please try using the command line in Terminal.app to check on the file.
1. Open /Applications/Utilities/Terminal.app
2. Type or paste in the following (with the trailing quote mark):
ls -l "
3. In HoudahSpot, select the file
4. Then select "Copy File Paths" from the "Results" menu
5. Return to Terminal.app and paste immediately after the the open quotes.
6. Type a closing " after the pasted path. You will have a command like this one
ls -l "/Users/pierre/Folder/file.txt"
7. Press Return
8. Terminal will show some properties of the file (permissions, owner, file size, etc.) or show the file path with the message "No such file or directory"
Is the file really missing?
HoudahSpot searches the Spotlight index. A possible explanation for this odd behavior could be that the index got out of sync with the actual file system. You may want to try to re-index your drive.
1. Go to System Preferences > Spotlight > Privacy
2. Add your startup disk – not just individual folders – to the list
3. Wait a bit to be sure the old Spotlight index is deleted
4. Remove the startup disk from the Privacy list
5. Leave the computer running overnight to rebuild the index