I routinely use the "rebuild index" tool as I am always wanting to make sure, well, the indexing is accurate.
Generally, rebuild index, should delete and rebuild all your resource indexes. Yes, this can take minutes to hours.
I noticed in 37.0.533 (admittedly I never checked before) that some resources seem to want to be reindexed.
For example, after doing a rebuild index to completion, I then typed the reindex took and it drops down 6 or so resources listed as
Reindex The Gospel Of Luke
Reindex Anacharsis(Greek)
these are just 2 of the 6 examples.
I had an expectation that the Rebuild Index would have resolved those, and all the unindexed resources.
DaveG