A boy dowses for water then "drills" into an abandoned oil drum
I believe this was at least 4 books that I read pre-1986. I'm recalling too many things about it that may be contradictory or from other stories, but:
I think his name was Henry (unreliable), and the neighbor girl he befriends might be similar to Midge (not the dog & boy from "Henry & Midge"!)
He found a cool place/building, explored it some, and some adult wanted to accuse him of breaking & entering. A woman speaks up to point out that there was no breaking involved, as it wasn't locked. She probably was his caregiver/aunt. This is how the boy discovers that this woman is/was a lawyer. Likely 1st book in series.
He's staying with relatives/friends of his parents while the parents are overseas (US State Department?) I recall the setting might be New Jersey.
Another book, he dowses for water, gets a signal from the forked rod, then "drills" with a heavy metal rod that simply is lifted and dropped repeatedly until it breaks through into a buried oil drum/tank.
There might be a book in which he & the neighbor girl join adults on a road trip. Consider this unreliable.
AI searches kept misleading me to "The Great Brain" & other things that weren't right. Thanks for any help.