Why PPD test in patients without M.tuberculosis often slightly positive?
The question asks PPD test in patients without M.tuberculosis.
It does not say if the patient has previously had infection with M.tuberculosis.
I think this question implies that the patient has never had before infection with M.tuberculosis.
I am not sure how it can be in reality possible for adults, since M.tuberculosis spreads in air.

The reason for false positiveness can be that assays that use PPD use unspecific antigens, in contrast to IFN-gamma and second-generation assays.

The other reason is that once you get infected you always have some degree positive skin test reaction. 
So to say patient without M.tuberculosis is faulty in a way because I think most people get exposured to this bacteria at least little in life.