3137.mp4 -

Did you run exiftool 3137.mp4 ? Often, flags or hints are hidden in the "Comment" or "Artist" tags.

Briefly state what the goal was (e.g., "Find the hidden flag within the video file"). 3137.mp4

Use file 3137.mp4 to confirm it is actually an MP4 and not a renamed archive or polyglot file. Did you run exiftool 3137

A quick summary of the trick (e.g., "The flag was hidden in the LSB of the audio channel"). 3137.mp4

If the video looked normal, did you check the audio track? Mention if you used Audacity or Sonic Visualizer to look at the Spectrogram for hidden text.