Thanks for all the replies.
This really has me stumped. I know there's no oil inside the cable. The outside jacket just appears to be degrading. Has occurred to some degree on all probe cables but a couple have been a bit more. No harsh environment, mostly just my home bench. The scope itself still looks like new. Shortly after original purchase, I bought the protection and transport case, model RTB-Z3, made just for this scope and 3000 series. kind of expensive, but I don't regret the purchase, works really well. All the probes and a few extra cables and a differential probe fit in there. I thought that might be made in Germany. It has a unique smell to it. I used to roll all the probe cables and slide them inside front pouch, but after I noticed this wetness on the cables I put them in baggies first. I don't think it's a reaction to case materials, none of the other cables inside have this issue. When I first noticed the issue, I wiped them down with 91% Isopropyl, then water. R&S Rep recommended same thing. Few photos from storage baggie.