There is a whole housing that the thermostat outlet mounts to. There are actually a few places it could leak around that area.
http://www.cygnusx1.net/supra/librar.../co/CO_02.html
With the hose clamps you are using (very common), they tend to start seeping in the winter. The rubber hose shrinks a little. I always take a few minutes after the first cold weather to snug them all up a bit.