There are various fonts with such restrictions.
Are you able to figure out which font it is, name of the font?
Run a Preflight. Create a new Preflight profile. Turn on Type > Font Types Not Allowed > Protected Fonts.
Does it show up in Type > Find Font...?
You can also do a Find/Change GREP query for .+ and set the Find Format to look for the font. This would find whitespace and paragraph returns using the font that are not obvious otherwise.
If it doesn't show up using either method, odds are good it's in an unused style.