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?
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?
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?