Looks like you've just been really unlucky, I think that the 'Go find this hideout and destroy it' quests may spawn a hideout, though they may just use a preexisting one.
The best way to find a hideout is to examine the patrolls, they don't go too horribly far from the hideout say that, for example, you've found one patroll around village A, and another around village B, and a third around castle C, and the locations on a map happen to form a triangle, or can be seen as points on a circle, then you should go to the center of that shape and search around there.
Remember to look for bandits that are 'Patrolling around hideout' (Or campsite, or whatever), bandits chasing after random parties are not very good indicators, also remember that if the bandits chase or get chased by something a long distance, they will, once finished with that, immediately resume the status of Patrolling (can't remember, they may go to travelling for awhile, but still this should be true) , but won't necessarily be near the hideout, so if you track them, they should head for their hideout for awhile before going back to their normal routine. I'm not exactly sure on the last bit, I haven't tried that, usually I spot a few parties and then go to wherever their center would be.
I've always assumed that the bandits patroll a roughly spherical area a certain distance from the hideout, and it's worked well for me.
Hope this helps, now run them down!
