I ran out and got some UV dye and put it in the tank with some water.

I haven't noted anything around the temperature sensor - the steam I would see does come from that side but the dye was showing up near the throttle body. I didn't run it long enough for the tank to empty itself (yet) but I'm thinking it's the vent pipe that branches off to each upper head.

Should there be something else I am looking for in that area, like the transfer pipe weep hole? - hopefully not that