Use grep. That’s what I do. From the UNIX or Mac Terminal command line, cd to your buddypress installation directory. Then:
`grep -nR ‘bp_after_signup_profile_fields’ ./`
That’ll tell you all the locations of all the instances of that text.
In the case of this particular do_action, I am pretty sure it is just there for the use of other plugins. I don’t believe it does anything in BuddyPress itself.