Why aren't we referring to Yamada as Yamada-san in the following sentences
taken from the answer to a recent question (which took them from site at:
<http://www.e-hoki.com/column/current/68.html>) where you are talking to your
boss (課長）about a colleague of equal rank?

その件は、山田に御説明しました。…謙譲語Ⅰ  
その件は、山田に説明いたしました。…謙譲語Ⅱ（丁重語）

The first sentence is wrong anyway because the ごーon 説明 is unwarranted (see
original question - link below) but in either sentence is it not correct to
refer to others with ーさん?

In the second case the speaker could possibly be treating him as "part of his
group" but, given we are talking to our section head, I would have thought we
were all in the same group?

_My thoughts:_  
Is it possibly because we are using 謙譲語 with our 課長 we therefore have to refer
to our 同僚 such as Yamada without the さん?

If this is the case then if we speak in neutral Japanese to our 課長, should we
keep the さん？ ie:

山田さんに説明しました − correct  
山田に説明しました − incorrect (or just a bit rough?, because I don't think it is
unusual)

Recent question: [What distinguishes 丁重語 from other honourific
forms?](https://japanese.stackexchange.com/questions/11692/what-
distinguishes-)
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question
politeness
Dropping ーさん when referring to someone humbly with 謙譲語
The inside-outside relationship trumps and the speaker is still humbling
themselves relative to the listener by using 致します. Even if you are talking
about your boss to a customer for example, you would not add ～さん to you bosses
name as the inside-outside relationship trumps the internal relationship in
the organisation.

I've seen a lot of Japanese people (and many that really should know better)
get this wrong as well when they speak in business situations.
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answer
If you look at the site you linked, you'll see that it explains that the first
sentence you mentioned is actually incorrect, because the assumption is that
山田 is your coworker and you would not be elevating him with 謙譲語Ⅰ.

So, yes, if you're assuming 山田 is not your coworker but in fact someone
important, resulting in it making sense to use 謙譲語Ⅰ, then you would likely
also want at least さん on his name for it to be a proper sentence.
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answer