Foxing

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I have heard the term "foxing" being applied to some brown spots on notes.  I found the following definition of "foxing" at Wikipedia.

Foxing (from Ferric OXide, or from the fox-like color of the stain) is the effect on certain papers of the oxidation of iron or other substances in the pulp or rag from which the paper was made. It is responsible for the spots and browning seen on old books, documents, postage stamps, and so forth.

While unsightly, and a negative factor in the value of the paper item for collectors, foxing does not affect the integrity of the paper.

My question is - is foxing a spontaneous event or did the note - at one point - get wet or something else happen to it for foxing to start?

Foxing does seem to be a particular hazard for notes that have been in the Tropics - BWI issues of Canadian chartered banks for example.

One of the first notes I bought on EBay was a 1935 $ 5 English. It was advertised as having foxing on a small portion of the top right corner which is visible on both sides of the note. The picture is gone but the description is still there.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=ADME:B:EOIBSAA:US:11&Item=8341704003

I paid $ 300 CDN for the note. It also has three small pinholes and was fairly graded VF-EF if the flaws were not present. I felt I got a good deal as the note would be in the range of about $ 600 otherwise.

This is the foxing I referred to in the previous post

Front

Foxing

Rear

That doesn't really look like foxing to me.  It looks more like something -- grease? -- got on the note and made a stain.