I think the familiarity of playing a piece on a particular instrument will usually fade with time if not played in a significantly long stretch of time.
However, one can always pick it up again. After a few moments of 'refamiliarizing' with the motions and feels of playing the piece, the usual prowess will come back.
Yes, I agree with you, when not played for a long time, the familiarity of playing the piece fades. The easier compositions are not a problem to refresh the memory, the harder the piece is, it takes longer time to play it again (well, or in a decent way) :))