That is wrong as far as i'm aware mate.
I wasn't there tonight so can't really comment on the decision until i see it, (although it did sound bizarre). As i'm currently 'studying' the laws of the game in preparation for my Referee course i thought i would just clear this up.
Extract from The Lawsof the Game 2008-2009
Law 11 Offside.
A player in an offside position is only penalised if, at the moment the
ball touches or is played by one of his team, he is, in the opinion of
the referee, involved in active play by:
• interfering with play or
• interfering with an opponent or
• gaining an advantage by being in that position