Do you recall the due date for the remaining principal balance on the note? Wasn't this the note that was paid down by 50% and the remaining balance due date extended?
Maybe the payment has been made and they are holding the paid note so they don’t have to do the paperwork work twice when the company and patents are sold. Kind of like paying off the loan on a car and holding the secure note marked paid in the bank deposit box because you know your about to sell the car soon so why pay to change the title.