Dear Mr. Gross,
With respect to some of the other commentators, the lease may not have expired. Do you know if it was pooled with other lands and there is not a well currently drilling or waiting on a pipeline connection or division orders to be sent out? If the primary term expired just a short period of time ago, a well could have certainly been drilling through the end of the primary term. If a well was drilled through the end of the primary term and was a dry hole, the Lessee might have the right to drill another well. Did you examine that portion of the lease form?
You might find this blog post illustrative:
To make certain you maintain a clear title, send to the Lessee a Demand for Release letter. And get the Release. Mr. Bigfoot uploaded a sample for you. [Thanks, Mr. Bigfoot] Landmen running title love to see Releases of leases. It puts their mind at ease.