I'm leaning towards wear/abrasion
I'm leaning towards wear/abrasion
Given that there are also "dimple" dents in the cassette chamber as well, I'm leaning towards wear/abrasion rather than fungus. Also pointing away from mold/mildew is the totally smooth-clear-clean surfaces on the rear area around the shutter port - and the shutter curtains look great, and they're usually the worst area for mold. Mold/fungi need something to "eat" - like the silk proteins in the curtains. Hard to tell from the screen shot but the top end of the cassette chamber, around the shiny prong-thing, appears to be "nice".
Or (here's a thought) a previous owner hand-rolled their own film and used something like goopy medical adhesive/sticking plaster as labels on the roll.
Inquire of the seller - if this is on the 'bay, seller either acknowledges a flaw up front or Buyer gets a straight go at return/refund. Simple question: Seller, does this stuff wipe off with a damp tissue?
I have, at best estimate, about a dozen S2 bodies. The only funkiness I've ever seen tends to be in the viewfinder, at least in mine. Auction-site photos of blotchy shutter curtains turn "mint, like new" into "paperweight". Final word: find a Nikon S2 you love and stick with it forever.--alfredian