Why does my water heater's drain line rejoin the outlet pipe?
What you describe sounds like a Gravity Fed Hot water re-circulation system.

It's designed to keep hot water available at the kitchen sink without having to flush cold water out of the pipe first.   The trade off is waste heat because the line out to the kitchen sink in your case is always kept hot.

Sometimes these systems have a pump, but they don't need to especially if the piping is above. As the water cools it becomes more dense and thus heavier, which creates the circulation.  

The following diagram is shamelessly stolen form the internet. 



Strictly opinion follows: it seems like an experienced plumber would recognize this method, it's been around a very long while.