It is possible the exhaust manifold bolt is seeping, running down the head and dripping off. I would pull the rear bolt add sealant and replace. To me that would be the simplest place to start.
I agree. Since it's a small leak it could be running down from the exhaust stud and you just can't see it. I've seen that happen a number of times. Pressure testing it while cold is a good idea. I've found more stubborn leaks with my Stant radiator pressure tester than I can shake a stick at. I would be surprised if it is leaking from a head bolt unless it wasn't properly torqued.