Implied "no"

A little something that's bugging me. Where does the の come from in words like these?

井上 - a last name
幕内 - the top division in sumo
山手 - The Yamanote-sen, a train line

The kanji's readings don't look like they should include a の, but somehow it's forced in there. Why/when does this happen?

I don't think there's any hard and fast rule, but it tends to be older names that have it. It's a "we-don't-feel-like-writing-it-anymore-and-everybody-knows-it-anyway" sort of thing, I believe. I think it doesn't happen enough to worry about, so you can just learn them when you come across them. I think you've got the major ones already anyway. I believe the same sort of principle is in effect for words like 手続, 取引, and 取締役.

By they way, when I first saw the title of this thread, I thought it meant the "no" that Japanese people won't tell you but that you're supposed to infer, for instance from "maybe."* Heh, I was way off.

*Yes, I'm aware this is a stark (over)generalization.

I've seen implied okurigana many times though. Mainly, it's taking the okurigana of a verb's stem and omitting it. That's also what's going on in all of your examples.

取（り）組、締（め）切（り）、小包（み）、申（し）込（み）用紙, etc.

Right, and I was saying the same principle applies to those as to the "missing の."