It is impossible to tell anything without having the original file handy.
You may share the file with us, you may use Google Drive or Adobe Send or Dropbox to share the file. What exactly did your team do during the preflight check?
Thanks for your reply.
Due to confidentiality I am unable to have the files provided however let mer revert again to you on this.
And for your kind info we do not have any chance of even doing pre flight in this file where this mysterious line had already appeared.
So if you can advise where this line come about in really appreciate it.
We cannot guess the cause, this is not a standard problem. But we can suggest some actions that may help in troubleshooting.
1. Take the file with the red line. Return it to the original PC. Does it have a red line in the copy that is returned?
2. Consider how the file was moved to the preflight system. Do not use email.
3. Compare the exact software AND version used to view it originally and to view the file for preflight. What are they?