As Link says, anything external can cause this problem. This includes libraries, hotlinked modules, xrefs, and external drawings.
When you go off-network, you will always get warnings about such things not being available, which is OK.
There is an intermittent issue, which I have observed off and on as long as I've had a portable Mac, where AC just won't get the message that it's off network, and it's OK with you that it is. It wants to keep checking no matter what. The behavior I've seen is that when you switch apps and switch back to AC, big hang.
Some projects have it and some don't, and it comes and goes.
The one piece of concrete advice from GS involves a internal preference setting related to multi-platform networks. Look at the link in this thread:
http://archicad-talk.graphisoft.com/viewtopic.php?p=91712&highlight=disablecrossplatformmountingfeat.... I wrote it up here:
http://www.onland.info/archives/2006/08/disablecrossplatformmountingfeatures.php
Cache library is a feature so sick with bugs over the years that I would never use it.
A location manager type of feature would be very welcome, considering how many external files and resources are typically in use. I'm sure there's a wish around here somewhere for that.