If you want to search for a card across all your boards by a particular text containing in its comments, you can use the Advanced Search.
Click on the Magnifying glass at the upper right side of your board interface to open the Advanced Search.
Click on Refine search criteria to open the Refine search criteria panel.
Select your search criteria: e.g. Board and Comments -> click on Add.
Define your search criteria. Select an option from the dropdown menu: "Comments contain" -> type the text you are searching for, e.g. "Follow up" -> click on Search.
Use comma to search for multiple words at once.
All cards containing this text in the comments will be listed in the results.