[VedalkenBear]

Pro: The computer seems to select the lowest-numbered province you control, subject to the following criteria.

1) The province has not been manually searched.
2) The province has not been automatically searched for the path in question (i.e., it won't Augury a province that has already been Auguried).
3) The province currently has fewer than two sites located within it.

AFAIK, these are the only criteria it uses. It will not, e.g., preferentially search forest provinces with Haruspex. Also, it will cheerfully search your capital province if you are a nation with only one site located there.

Generally speaking, if there are no provinces that fit those criteria, it seems to default to searching your capital. That's a good indication that you may want to check for why it's 'not working'.