If a file contains ASCII character 1C ("File separator"), then it never appears in "Find in Files" results.
Searches within the document seem fine, it's just for "Find in files" that it doesn't appear.
To reproduce, create a file containing some known text, plus a 1C character. Then find in files for that known text. The file does not appear in the results.
Servicio de atención al cliente por UserEcho