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It's not mysterious. It's there to alter the sensor (usually radar) observability properties of low-observable (stealth) aircraft -- usually to make them more detectable.

Citation: worked on them.

What I'm reading in your comment is that the skin is put on to answer the question: "what would it look like on radar if it wasn't covered in radar-absorbing paint?", right? Probably a stupid question, but my first thought was, "doesn't making it more detectable defeat the purpose?"
When they want to know what an aircraft looks like without the radar absorbing foams and paints they do that during the testing phase, typically on a prototype that simply hasn't had the fancy stuff applied. Anything done to increase radar signature is typically the aircraft equivalent of the reflective belt.

I think the author's hypothesis about IR is far more likely and provides much more justification for why and old type (that they've presumably long since had radar information for) would be tested alongside a newer type.

> Probably a stupid question, but my first thought was, "doesn't making it more detectable defeat the purpose?"

Maybe you don't want your stealth aircraft to be stealthy during peacetime, to avoid giving your adversaries the possibility to get their hands on some valuable training data. For example, if you are China or Russia, wouldn't you place some agents with some antennas around the US Air Force training bases and compare the radio reception with the visual sightings? If the radio reception is not the same you'd see during wartime, all this exercise is futile.

More accurately, to make it more noticeable to sensors or alter it's signature (eg. the sine waves on the wings probably aren't just for looks).
Surprising they are still flying the F-117 in limited numbers even after its official retirement. It is such a cool looking plane
The shape of F-117 is absolutely out of this world. This along with the B-1 spirit are savagely alien aesthetically.
Guessing it’s been studied to death re radar so probably makes a good test platform
They've also been running this skin recently on Navy ships. There was a post or two about it on the specialaccess subreddit.