Then it’s just a matter of following them.
When they are travelling they are returning directly to their hideout. If they are patrolling they won’t lead you to their hideout either, but if you follow a patrolling group of bandits they will eventually change to travelling. If they’re chasing someone or being chased they won’t lead you to their hideout, ever, so don’t get close enough to make let them notice you. They’ll be either running from enemies (usually you, if you get too close), chasing enemies if they see someone weaker than them, patrolling, or travelling. When you see a party of bandits mouse over them and you will see what they’re doing.
When you’ve done that you can either scour the area hoping to stumble over the hideout, which is probably not going to work, or you can follow bandits back to their hideout.
As bandit parties on the map are eliminated new parties will be spawned near the hideout so after a week or two the bandit hideout area should have noticably more bandits. To find bandit hideouts the first step is to find where there are most bandits.