Penny - if you're not sure, chuck it. Can't take any risks. I have to say, I still use some old colourings and had no issues with them. I wouldn't think they could get toxic or anything, but might not colour properly. Certainly don't see why you can't use them on accents that won't be eaten. Try colouring a small amount of butter cream or fondant and then eating it - see what happens!
Indeed Pat - I wouldn't use them for a paying client, only personal.
You have a good point Penny - she could have used a stick that had butter cream on it for example and stuck that back in. I don't think you're paranoid at all.