My apologies for not answering this query sooner. I needed to ponder on it and having done so I would say no, I don't believe title insurance covers hidden oil tanks. I write coverage in many rural areas where underground oil tanks are on practically every property. Though I've never had a claim or this question come up before, I can't see any reason for title insurance covering any matter related to a hidden oil tank.
That doesn't mean that coverage does not exist, it's just my opinion. If you are faced with a real life situation involving a hidden oil tank and you have an owner policy, just contact the title insurer and make a claim.
If any readers have experience with hidden tanks and title claims, I'd be happy to receive your comments.