From the TA (placed on indexers, or standalone search heads), the "Update monitored list from REST" saved search runs just before midnight to populate the monitored_indexes.csv. The same host (still the TA) then uses that list a few minutes later (just after midnight) to run dbinspect over each index. This data is stored in the summary index.
Later, the main Fire Brigade app on a search head referencing these indexers (or in a single, standalone instance, where both the TA and main app would be installed side-by-side), runs a search called "FB - Host Index Cache" to populate that lookup. This is used for dropdown menus and the like.
The list of host / index combinations in the lookup is driven from data populated by dbinspect. The logic of the search is "give me the set of dbinspect records from 'today' and count by host, index". If the "today" part of that equation is confused, because of time zone issues, it could result in an empty list. You can always click on the "run" link of the saved searches page (Settings > Searches, Reports and Alerts) for the search in question (here it's "FB - Host Index Cache") to see if it finds results as expected.
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