Air condition leak?

[mxxxs]

Hi,
I have a 2008 3.0sd, and i have a strange problem with the aircondition.
It seems that i have a leak somewhere, but nobody can't find where is it.
I have checked the car to 2 different dealerships, and maybe more than 5 other shops.
Everybody says that there is no leak, as it passes the vacuum test, And they cant find any of the dye that they put inside.
The strange thing is that, as along as i use the car daily, there is no problem.
When I don't use it for a day or two, then i it is almost out of refrigerant.
Does anybody have a clue about this?

Thanks