You are right, it is not a macro.

Your template is a mailmerge document and the content in question is a merge field which tells Word to include the underline before the merged text in the event of the field NOT being blank.
MERGEFIELD SigTitle1 \b "___________________________"

Did you put the \b switch in the field or did you inherit this from someone?

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia