Well, this is a very annoying problem:

My sim is going with the ghost hunter proffession, but it have happened two times, that when i have caught all ghosts, there is still this weird "mist" or what to call it, "the sign that shows that the house is haunted" so to say.

any ideas?

I have a ghost hunter too. Sometimes he needs to scan for ghosts on the veranda or at small places outside the bedroom door on second floor.

are there ghosts that aren't related to the mission there? or just to make it dissapear?

I have had ghosts appear in unreachable places--like in an unreachable attic or in an unfinished basement with no stairs leading to it. It really messes with the game. Sometimes I am able to teleport into the area using a cheat, but some attic spaces--I guess the roof is too low or something and it won't let me port in. Best of luck--I hope you find your ghostie!

another..... interesting thing that has happened, there was a couch wich i had to exorcise, but when i did it, my sim didn't move after 4 or 5 seconds, after a few tries, o couldn't do anything with the couch, except sit/nap etc, but the other sims at the house still act as if it's possessed, is there anyway to reset the house or something?

Every time I have played the Ghost Hunter Profession I have found that when my sim got to a certain level there would always this would happen

My sim would go to a house to do a job. They would deal with all the ghost. After they were finished, the spooky music was still playing and the mist was still in the house.

I have found that every time this would happen, to fix the problem, I would save and quit my game. I then would restart my game, and when I did problem fixed. I have done this so many times (different sim/different games) I can not tell you the exact number, but when I saved and quit, it was fixed when I went back to it every time.

I'm gonna try that at some point.