"Find Includes" works only within the space where the MultiExcerpt is located
"Find Includes" does not find any includes that are outside of the space in which the excerpt is located.
STEPS TO REPRODUCE:
create a multiexcerpt in space 1
include that multiexcpert from a page in space 2
in space 1, edit the page containing the excerpt so you can click on "Find Includes"
it will NOT find the include from space 2. If you have any includes in space 1 it will find those.