Possible leak from upstairs window into downstairs window
After several years of fighting this issue and asking many experts, I finally have a solution.  I attached a garden nozzle with a high pressure setting.  Then I sprayed the windows all over the corners and edges.  Nothing.

So I started looking at the bricks.  I found several places that looked like holes.  I sprayed those.  FINALLY.  Water came into the house.  The mortar came loose from the brick and caused the leak.  I'm going to plug it up with Quikrete Mortar Repair Sealant.

If I had a buddy I would have also tried a leaf blower.  I figure if water can get through then so can air.  I was going to push the leaf blower into the hole in my window then go look for a place where air is blowing out.

I credit Dan Schweihs at The Window Dog for helping to finally reproduce the problem.  And I'm thankful he did NOT blindly sell me windows that would not have fixed my primary concern (as several other window companies did).