It seems strange that dictionaries don't mention much or emphasize speed or shortness. That's how I know and use the phrase. It's not so much exactness as it is a matter of quickness, taking (considerably) less time than expected. The best dictionary definition I could find addressing this was Longman - a learner's dictionary. Longman lists the usage as a separate entry, that is, as an idiom of its own. Such "learner's dictionaries" often include aspects of a word's or phrase's meaning that native speakers just take for granted.
Surprisingly, there are even a few instances of "hours flat"
That doesn't seem surprising to me. It depends on the context. If in the second example, say, the records were supposed to be incredibly difficult to find, no clue as to where they might be, scattered about in various offices, countries, etc. and it was expected to take weeks or even months to locate them all, then "In three days flat I had located all the records" sounds perfectly normal.