You describe that the scab is located on the upper part of the anus. In the article sent to "Bella and Cody", the original poster, it states, " the anal sacs beneath the anus at roughly the 5 o'clock and 7 o'clock position". This appears to be a different location than where your cat's scab is, but it may still be possible that the glands are naturally emptying themselves and the discharge is sprayed "upward" leaving the evidence higher up, or, when the cat's tail is down, it smears the discharge and it gets deposited higher up? Just a possibility.
When you remove the scab, does the flesh below seem raw and sore? Or is it just a "will-not" or "cling-on" as I like to call it! of some other material? Either way, get the vet to look at it again and check the glands while he/she is at it to rule out a gland problem definitively.
Boy, we sure spend a lot of time looking at our pets' arses, eh?