I think it's usually reliable, but I have also come across situations where it tells me that another user has changed it in the repository, sometimes immediately after checking it in (if my memory is correct). The icons are explained in In the help file:
There may be several reasons why some of the metadata associated with a model could change in the repository, even though nobody else has updated it, such as checking in a related model. So, if you see the yellow exclamation mark ( ! ) that just tells you that something was changed on the server, and it might have involved a human being.