I think for motion verbs like 行く、来る etc. when said as 行っている or 来ている for example (I don't know the technical terms so bare with me here)
Is like saying "went and is there now" or something along those lines
So for example:
彼は図書館に行っている - He went to the library and is there now.
I think it's the same with all motion verbs like 来る、帰る etc.
Perhaps somebody else will be able to explain in more technical terms than I can and if I am wrong then I would like to see somebody else explain, otherwise I would have had the wrong idea for a while now.
