@MrJohn91; Have you owned the car from new? If not, there’s always a possibility it might have had a repair or new paintwork on that panel before you bought the car and the panel wasn’t cleaned / prep’d adequately before being repainted. Could it be that it’s always been like that and you’ve only just noticed it?
Is the panel join on the left side of the picture the join between the rear bumper and the rear quarter panel? If so, then I’m pretty certain the reverse side of that part of the rear quarter panel isn’t exposed to the elements (it’s actually inside the car and you should be able to get to it by removing the interior trim in the boot). Therefore, IMO the issue you’re seeing in the circled area of paintwork in your picture is unlikely to be corrosion from the reverse side of the panel as it’s not exposed to water, road salt, road debris or other external elements that could cause corrosion.
If you know anyone with a paint thickness gauge, you could get them to measure the paint thickness on that panel and compare it to the paint thickness on the rest of the car. Any significant differences in thickness would
help to confirm whether or not the area with the paint imperfection has been repaired / repainted.