No, one can never be sure. That's true.
So it's not like you can say "no, there are no more lost tapes around". I'm pretty sure some things eventually will turn up somehow, somewhere.
One of the latest discoveries is supposed to be a very early recording of "Rock Around The Clock".
It's probably a poor sounding live performance from the Louisiana Hayride shows.
There's also a rumor about a version which is a medley: "Good Rockin' Tonight/Rock Around The Clock". Also from The Hayride Shows.
But considering the source of these recordings (like f.i. Hearts Of Stone, which also was a so called discovery) the sound will probably be very poor.
And of course there's the on going rumors about several songs Elvis recorded, mostly private recordings.