I agree with Wayne and Wojtek on the 'sheen'.
That 'Red Dot', as Wayne pointed out, is the cover over the ground power unit socket. The cover itself is a round 'flap', not unlike one you might find on the fuel tank of a car, and this is set within the circular mounting. If looked at straight on, it appears as two circles, one within the other. The cover, and often the circular mount, was painted red, to be immediately visible to ground crew. In the pic, it is possible that the cover is missing, or not fitted in the first place, which was common, but the area would still be painted red.
It should be just visible in the attached pic of my 1/48th Dora, inside the black cross. (if I've posted the right pic!)