If it is a oil tank. Your neighbor does not want to know that. Because if he does know that it is an oil tank, he will legally have to disclose that, if he goes to sell the property.
Just because a oil tank is decommissioned, it does not mean all that much. An oil tank may have leaked before it was decommissioned. If so a picky buyer may want the soil tested. And if comes back contaminated clean up costs can be huge. Really best that he not know. Pave over it and forget it, is what I would do.
If it is close to the foundation, oil tank vent is a good guess.
Problem is though, could have been a house on the lot before his was built, they just didn't worry about stuff like that much, just left stuff where it was and built new.
