could be either, there is no set rule - usually there is a co car handbook which sets out this kind of stuff
eg in ours it says that the return will be arranged, we have some people who leave and leave the cars at the office on their last days and some - who have been made redundant for example and kept their car for the redundancy period, those ones we tend to get collected.
Its helpful if you have a decent relationship with the co as they may be inclined to make it easier for the returnee, i've ended up driving people back home before.