You get this appearance when heat is applied to the foil wrapper either in a manufacturing defect or reseal. I ran a few experiments on old wrappers I had to see what it would look like. In particular 1999-2000 USA packs.
From my experimentation, it is incredibly hard to do a reseal. For starters, most packs do not open cleanly to where you could even reseal it.
Then you need just the exact temperature to heat the wrapper without melting it or very obviously distorting it.
One thing I have noticed is that if I apply any heat, even without any noticeable effects of resealing, the pack will start having a much more iridescent appearance. You can look at the pack in light and see it right away.
I would guess yours is just some kind of manufacturing defect. I own a few which I believe are defects and look the same.
Not super scientific but I heated up a flat knife, and pressed it into the vintage seal. There may be much better methods.
Too much heat:
Basically no heat, top left has the iridescent/oil slick/rainbow appearance and crimp lines fade slightly.
