I am currently in 'discussion' with iTunes about this very issue. It's happened to me at least twice before when I redeemed a Gift card and didn't use the full amount. A couple of weeks later... the 'credit' was gone!
I also question as to why iTunes do NOT show gift card redemptions... VERY sus imho!!! Why dont they??!!
I have asked iTunes to provide me with a list of all my transactions for this year so I can see when, where and how my credits were used. So far the best I got was "check your account balance". They have said they will look into it from another side (ie. not via their customer support) but I will give them a week before I go to Fair Trading and make a formal complaint.
I have provided them screen shots, dates, my purchase history etc. and the numbers just dont add up. Either my credit balance expired (in the space of a couple of weeks) or someone is skimming accounts - which is on the iTunes system. Either way iTunes have a case to answer!