I am not sure, whether i really got all the loot. I definitely tried, but most often i miss some well-hidden trinkets or coins. I blackjacked some AI (including a sleeping one) and might have been almost seen by the elite guard (he barked but did not draw the sword). I tend to quicksave a lot and quickloaded a few times after alerting AI (i generally load if AI starts screaming or drawing weapons). Most often, i get a non-zero stealth score. If a mission has internal leaks, that score can also be 2-digits because then, i tend to alert some innocent bystanders on another floor when running through already emptied hallways.
This was the only time i ever got an all-zero stats screen. The stats screen of the next mission i played showed plausible values.
Thanks, by now we can savely reproduce this behavior. It is FM specific: It happens on Closemouthed Shadows (CS), but not on "A New Job" and the bug is independent of loot and stealthscore. The problem is that in CS, the game wants to show the objectives screen before showing the mission stats, and the objectives screen (correctly) resets all mission data. While NBohr has added a workaround, I am still puzzled as to why the FM wants to show the objectives screen in the first place. I am still investigating this issue as it might point to a bigger problem... Who knows?