If I had more information about the campaign I could give more specific suggestions.

But typically, when I like to run a campaign and there is a major story branch I work it like this:

I create 3 possible places/main choices that can be made so it gives the players the ultimate choice of where to go/what to do, but I'll still be prepared.

Possible ideas for the scenario:

1. The characters hear indistinct chatter through the walls, maybe a vent or such. On a high enough perception, one can overhear them talking about said location of interest. Perhaps a compound of survivors preparing for the worst at a hardware store, using the readily available supplies to fortify a wall around the building. The slavers obviously wanting to go there to capture more people. Your people show up and warn them of impending danger, help them defend the mid-way built compound, and are rewarded for their actions.

2.While escaping your group runs across someone looking for insulin for their diabetic kid. They help them find some, then they take you back to where they are holding up at, which happens to be a somewhat isolated farm and maybe this is where they decide to try and create their "base" for the next few campaigns.

3. One of the slavers who was inducted into the ranks of the slavers through sheer force and intimidation has compassion for you guys. Perhaps someone with persuasion or seduction is able to have them assist you with the escape because they are privy to the guards posts and routes. Before leaving the place she slips a note containing coordinates to an emergency cache location (which the players may need assistance from someone else or equipment from an old salvation army store to figure out where.)