Looks like a popped abscess to me as well- i can't recall which rat I had that had that but the drak scab sure looks familiar to me. I believe that dr Gold one of the MD rat vets - ended up draining it and placed the rat on the TMZ- SP something - a combo AB that saturates all tissues -
I know in dogs they have similar stuff happen when seasons change - and they call them hot spots the dog will lick it if they can and then it becomes an infected abscess. We trim the fur and then place a warm black tea bag over the area- the tannins in the black tea draw any of the infection out and dries it out alot quicker - Not sure if that would work on a rat - but I'd feel comfortable trying it - Then rinsing off the rat if its in an area they can lick-
there is a great product out there that has been used on exotics all types called HealX and Soother plus- By Harisson I've used it with success in a number of differing treatments - and it is completely lick safe -
Have actually used most of these products including the Booster which is great for myco prevention - Of course the rats that love it are usually the ones that don't need it - But there is a thread online somewhere about myco and booster being used and documented as keeping myco infections at bay without having to continue to rely on life time AB .
http://www.exoticdvm.com/heal-x
I think if you put Booster in the search here you can see those that felt it helpful - I know that the HealX has been helpful here- I have not been proactive about the Booster myself- just too many things on the to do list but I feel comfortable using it .