Why do filesystem indices get stale? The OS can track what files get accessed, isn't inotify watertight?
I know that Windows has some equivalent stuff, that the antivirus uses to disallow opening files before they got scanned, so I'd wager that API is functionally required to not miss any changes.
Yet Windows indexing is one of the worst pieces of software in existence and frequently gets out of sync.
Even with inotify failing, if the system is plugged in and idle, it could quickly do a scan for files with update date > last known indexed time and index those. Like once per day maybe. Wouldn't take much load or memory, IMHO.