how to find hidden formatting before saving as epub

My doc keeps getting rejected for publication due to ‘hidden formatting’. Suggestions are to find hidden text boxes. I’ve looked through and can’t find anything. Can anyone help, please?

Call up the Navigator (F5) to see if there are any text boxes.

The “text boxes” mentioned may actually be meant to mean “text portions” which are hard formatted.
Strict format specifications should be based on predefined formats on the paragraph level, and probably also on the character level. No hard formatting superposing this should be allowed then.
What happens with your layout if you ‘Select All’ and then ‘Clear Direct Formatting’ (Ctrl+M)?