No, it really wouldn't matter what type of cache it is. If I ever decide to go back, I'd point and click on it to get the details. Also, blue frowny would jog my memory and I could go back and see if someone after me found it...letting me know "I just missed it!"
My proposal is to leave the cache type icon the same, but change the background square to a different colour, red, blue, whatever. That way, you still see the cache type but also have the visual cue you have registered a DNF against it.