Supposedly theres an old man here....
Fully explored the dungeon and... theres nothing more to travel...

Did the game bugged out?
Ok... the game gave me a message that my Thief sensed something...
A secret door.... NVM!
Its freaking weird that you have to look at a wall for a Thief to sense a secret door....

Theres nothing about this in the manual or in the game!!!!!

Not even Might and Magic is this bad with info.

Next Stop, a weird Tree

This does not seems fun at all!!

I got stuck again at the Great Tree Yap Gotz!!!

Finally, found a guide on STEAM about this game...

It turns out its totally random that your Thief finds secret walls!!!
This just makes the game almost unplayable! My Thief would sometimes detect a secret walls and sometimes it would not.

Its Random that you complete the game... man... I just cant see someone completing this game without a guide.
Im convinced that this game MUST be played with a completed map to have fun.

One suggestion: The light spells, according to the wiki, are supposed to increase the chance of finding secret doors. Try casting Eroma when you enter a dungeon; that should (hopefully) help you find secret doors.

One other random note: Any area that requires picking a lock is not required to beat the main storyline; if you don't feel like exploring such areas, you can come back later. Any event needed to progress the main storyline can be found without picking any locks. (The items needed to enter the final bonus dungeon may require opening such doors, but by that point you shouldn't have any trouble with the encounters.) Also, you will later get access to many optional dungeons to explore if you want, or to skip (at least until it comes time to open the final bonus dungeon) if you prefer.

Spamming investigate button on every dungeon tile in all directions is the way to go.

Like dtgreene advised, buff your party with light spells (Eroma) to increase door find odds.
Plus try to raise the Luck stat, particularly on your Thief.

Last but not least, study maps for suspicious areas.