My water pump was leaking between it and the block. I replaced it a couple weeks ago when I did the timing belt and now the pump is leaking again and of course I have to take everything off to get to it. The pump doesn't use a gasket but has a rubber seal. Either way, I could see residue on the block below the water pump.

The other thing could be the overflow bottle. It's on the firewall but the hose goes around to the front of the car on the right side. Maybe you'll get lucky.