Search many workbooks for specific text, then insert a text in adjacent cell

I have a folder which contains many more folders each containing many workbooks with multiple sheets.

I am trying to find a way to search through all of the workbooks for any cells that contain the text "Signed By" in them and when found, place text (a name) in the cell to the right of it.

This is working very well to put in the signature, however, the message box stating there were already filled in signatures pops up for all sheets although there are not any filled signatures prior to running the code

Thank you very much. How would I adjust to put in a signature even if that cell already contains text? For example, I were to make a typo in the signature and needed to correct for it or if the name needs to be changed to a different person?

This is working very well! I do have another question. Is there a way to create a condition that checks the header of the sheet for a specific text that will set the signature?

For example, if header contains "Dept 1" signature is "John Hancock, if header contains "Dept 2" signature is "Sally Sanders" etc etc

So if there are 15 sheets in the workbook but only 3 unique headers this will not work?

I am having a problem with this where the last signature is always used. In this case I am always ending up with the signature "Tessie Tester" even though Dept 1 or Dept 2 is the text in the header