Hmmm, I do not remember a game titled "Treasure Hunt" by Johannes Plenio, but then again I do not know every A8 game in the world.
On the other hand, there was a type-in listing named "Treasure Hunt" by Thomas Fischermann (Interface Software) released in the german magazine HC-Mein Homecomputer 6/1985, maybe this is the one...?!?
This Treasure Hunt game by T.F./I.S. was also published (without authorization) on the commercial collection "King Size" and you have the screenshot right here: http://www.atarimania.com/game-atari-400-800-xl-xe-king-size_19003.html