Thrill on the hunt a unit with hidden card to play at the same battlefield does the hidden discsrded

No — the hidden card stays. You never lose control of that battlefield during Thrill of the Hunt's resolution, so there's no window for the hidden card to be trashed.

Thrill of the Hunt banishes the unit and then plays it (as a chain item) to any battlefield; while the spell and the played unit are still on the chain, the turn is not in an Open State, so the cleanup doesn't strip your control of the battlefield (Rule 190.4.c). That means you can legally play the unit straight back to the battlefield you banished it from, and your facedown card there is untouched.

The only way the hidden card dies here is if you send the unit somewhere else and that battlefield was your only presence there — then after the chain clears, the next cleanup removes hidden cards from battlefields you don't control and trashes them (Rule 323.7).