I wish we had a uniform opinion/ruling for this. I see people using it many different ways. I've always RPed it as a 'summon' of sorts, since in NWN thats what it is. The mechanics operate that way, and I don't like to stray to far from the mechanics (as this can often leave room for cheesing). Sure, thats not what it is in standard PnP, but NWN has different mechanics.
In PNP, a familiar is not subject to a dismissal or banishment spell, but on NWN it is. (Of course, in Ravenloft Dismissal doesn't 'dismiss' it either, just out-right kills it, since we have the custom raising system).
I also see ALOT of times, when people try to use a familiar or companion in combat, that they 'dismiss' it right before it going to be killed. If its not a summon, you shouldn't be able to do this. The animal would have to stay there and die. So if we are to RP it as a non-summon, there needs to be mechanical support to enforce it, you know?
I would like to just think of it like a summon, personally. Pulling your linked animal from wherever it is in Ravenloft to your side, and then sending it back when you unsummon it. Mechanically it behaves as a summon, why not treat it that way.
In regards to this...
I have seen people play this out nicely be saying things like, "ooh, that's my cat.. she is invisible most of the time." or " I beter bring Bruno to the woods before we go in there, it might be dangerous."
Thats the sort of thing that I worry about when there is no concrete ruling. You can't RP abilities you don't have. A familiar doesn't have the ability to turn invisible.. where do we draw the line? Can I say my familiar's flying overhead and spying on people, since that
realistically -could- be where its at? Granted, most of the time their explanation is harmless (I.e. the whistling and calling and all that).. but once we start allowing more unique explainations, where do we stop or what goes to far?