Found coolant in driver side spark plugs holes, only !

Today I found my radiatro low on coolant, turnsout that my spark plug holes were full of coolant, but not the cylinders. No sign of head gasket problem, block test done came back possitive.
leak down test ok, only 60k miles well very well maintain. This happen on driver side head only. Any one go thru this ordeal before please advise. Mechanics are wary of any work done, that may not fix my problem. Please help.