It is possible it is only in 1 plug. and it could be a plug from the back of the engine. You should take all the plugs off and do the test.
But another question first. have you noticed coolant leaking or I should say being used. When my Upper intake went I didn't see an external leak but I did see that antifreeze was being used.
I also didn't see white smoke billowing out of the exhaust at first. I think my leak started very slowly and quickly progressed to a fast internal leak.
BTW I will add that if it is an upper intake manifold issue, you could do the job yourself. I did mine and I don't by any means consider myself a pro. The trick is to label everything.