The unspokens trait says that when he occupies a square in the lowest terrain we can use leadership and stealth.
This means he has stealth and as such he must find a square of hindering to be invisible.
I was being told in a match this means he is always in stealth if they're in the lowest level.

I think this is wrong please help

You are correct. He does indeed still have to find some hindering terrain to be all stealthy.

Being in stealth only means that hindering terrain blocks line of fire. There is not "being in stealth" unless hindering terrain is involved. Otherwise it would have to say "characters cannot draw line of fire to this character".

Right. If on the lowest terrain level he can use Stealth, and he's using it all of the time. But unless a LOF drawn to him, on the opponent's turn, crosses Hindering terrain he gets no benefit from Stealth.

Always quote the power it helps:

It says he can use Stealth which means he needs hindering terrain.

To be stealthed is meaningless since that is not a game term, where to Use Stealth Power requires hindering terrain.

On a related note, if the map is all one elevation then all of it is the lowest level. And highest, too.

So Unspoken + Cypher needs to face off against somebody with the Kryptonian ATA on a single elevation map. Just for complete amusements sake.

Well, most of the Kryptonians have Supes Ally TA anyway, so it wouldn't be super useful.