Even though you may not see it, there could be a hairline crack in the block. If you are planning on rebuilding, then the first order of business for your machine shop would be to clean the block and then magnaflux it to check for cracks.
Barring a crack, there are three other possibilities that I can think of. One would be seeping from the head gasket. The other would be a leaky freeze plug. There happens to be a freeze plug right above the throttle linkage bell crank. Freeze plugs would normally be replaced during a rebuild. The third is something that was recently discovered by another bolter (
see here ) -- the hole that the throttle bell crank pivot is in apparently goes through to the water jacket. There could be leakage around the pivot.