There have been a few posts by people who have been told that they may own oil & gas rights that they were never aware of.  Here is a story that shows how this can happen.  (I had previously posted this in a more obscure thread.) 

I searched a property in Pennsylvania, currently over 100 acres.  The O&G rights were excepted and reserved when the surface was sold in 1928.  The grantor then moved out of Pennsylvania, taking up residence in a state far away. He wrote one lease, but there is no indication that a well was ever drilled.   He never sold the O&G, and died owning them, in 1953.  By his will, everything was left to a friend and neighbor, in this state far away.  The O&G  was not specifically mentioned in the will but would have been included in the residuary clause ("all the rest I leave to...").  It appears that the O&G rights were forgotten by then;  this is not unusual--remember that the shallow oil fields in Pennsylvania were spotty and often quickly exhausted so that such claims were often regarded as worthless.

The friend and neighbor died 1976 leaving everything to his children, who thereby became owners of the O&G on over 100 acres of Pennsylvania, and they likely never had a clue that they owned this.  As far as I know, they, or their heirs, are still the owners, and still don't know it.  I did this work for a prospective buyer of the property, so no-one ever followed up on this.  It will probably stay like this until an energetic landman pursues it further.  Note that, even if the out-of-state heirs had a clue that they owned something, somewhere,  it would be very difficult for them to figure this out.