They don't disappear. At least they don't disappear from your wallet. They do disappear from the top of the message list, because IHub hopes more awards will be made to keep them there.
I don't know if Ihub takes coins for my plus one posts, but when I did do that with posts I liked, the post and coins disappeared a week later. One time it was 450 coins for a post I liked that someone kept upping coins for a different post, all gone a week later. ----------------------- Why do the coins disappear after a week? And doesn't Ihub take a percentage from those given?