I had that last week - turned out to be my gym flip-flops which had baked away in the trunk for a few days.
On a Windstar I had, there was a leak from the evaporator drain at the firewall which ended up nearly rusting a hole in the floorpan. You could only find it by lifting the carpet up. The jute pad was basically rotted out and needed replacing. I would look behind the carpet in both footwell areas to check if you can't find a source otherwise.