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Weird. I'd be very careful with heat, particularly since afaik the substance itself is unknown. Maybe kitty litter, oil dry, baking soda or something on that order might do something without potentially causing damage. Since the paint is original, personally, I'd be more concerned about collateral damage, although I'm sure it must be a very frustrating situation.
Maybe, instead of concentrated heat, extended use of low heat ? Hair dryer, portable light, etc...I have a portable light that cost about $10.00 that will absolutely fry your skin if you touch it. It is really too bright to use.
I forgot if you said this only happens when it is outside ? If so, the sun and heat is drawing it out of its hiding place. Maybe, if possible, leave the car out for extended periods of time. Cover the car except for the problem area. UV rays are what KILLS paint. I understand you wanting to protect the original paint. But, I think, IMHO, you could rub that spot the rest of your life and not hurt it. The only thing that will hurt it is UV rays or trash. If there is dirt or particles on top of the paint, when you rub it, you are GRINDING those into the paint. So, hose the area off first, or apply liberal amounts of water to the area and then rub.
Before I paint a car. I wash it down with Windex. Windex contains amonia and will clean any contaminants off the surface. If you don't and start sanding, you are just grinding those contaminates into the paint and they will come back to haunt you. You can use Windex in your problem area without hurting the original paint. It will only remove wax, etc... That is what I would try. Windex the bad area, expose to LOW extended heat source (sun, hairdryer, etc) Then, remember you will need to wax that area or car again.