Cyaoeu, bravo! Thank you for your beautiful explanation. If I may be allowed to add something, the line above is in one single sentence, whereas the one below is a combination of two separate sentences.

They are basically saying the same thing, but the first one is using 謙譲語(one type of 敬語). As the person above says, the first one is used by a company employee like when someone from outside calls to 課長, and they will answer like that. The second one is the expression of exact same meaning without 敬語。 This is probably not important, but I think they say more like 戻っておりませんinstead of 帰っておりません。

The first one uses the humble variant of いる (おる). This is what you would hear from a company employee, talking about their own 課長. You can use おる when you want to be humble about yourself or your in-group. (family, company) The second one uses the regular いる verb and is thus not said by the company employee.