I was wondering what the parameters were for finding network holes with the Service Area connectivity check. I had a hole roughly like the area shown in yellow. Then I added a crossing end segment, circled in red, to connect the one-way segments and the hole filled in on the next run. Network
This is probably a quirk of the Service Area polygon generation algorithm. The algorithm has logic to determine when to cover over a large area like the one in your screenshot and when not to, and it has to do with whether all the roads surrounding the hole are reachable or not. If they are, then the hole may be covered if it's smaller than a certain size and depending on the trim distance configured on the Service Area layer. However, if any of the surrounding roads are NOT reachable (which was the case when you had that dangling one-way street), the hole will not be covered.
In this case, I'd say it did what it should, right? It helped you find and correct a data error, right?