He accidentally found the secret mine entrance in the old hotel safe.
5 seconds sound clip from Scooby-Doo, Where Are You! - Mine Your Own Business (S1E4).
You can hear this line at 00:20:12.778 in the Blu-ray version of the episode.
Quote context
[...]
- So it was old Hank who was scaring off my guests.
- To force you out of business.
- That way he could buy the guest ranch, the ghost town...
- And, like, the mine for almost nothing.
- Then there must still be gold in that old mine.
- But how did Hank know about the oil?
- He accidentally found the secret mine entrance in the old hotel safe.
- And instead of gold, he found oil.
- He had the exact location laid out on a map.
- Then he, like, hid the map in the cigar store Indian for safekeeping.
- Too bad. We could have been partners.
- Now instead of being rich, he's in jail.
- Hey, like, what happened to Scooby?
[...]