I had the same problem on our 2005 Escape XLS. At the time it had 76,000 miles. My wife complained of a rattle/bang in the back of the car. I was shocked with what I found. The passenger side was very bad and I even found a few small holes on the driver side once I got the interior removed. In my opinion the damage was caused by moisture/salt/debris finding imperfections in the under coating and was then trapped between the undercoat and the body until it rotted away. This happened in April 2013. I did a complete 4 wheel brake job and tire rotation in October 2012 and there were no signs of imminent failure because all of the undercoat was still in place, even if there was no metal behind it. I was chipping away undercoat during the repair that had no metal left behind it at all. I did all the repair work myself, making my own patch panels and the better part of 20 hours cleaning, cutting, welding, and grinding. The repair may not be body shop perfect but the price was right. On top of it all, a squirrel got in through the hole overnight and chewed on a trim piece and the center rear seat belt
Moral of the story: Your escape may be rusting away and you might not even be able to see it unless you remove the rear interior panels.
It is not accepting the links to my individual pictures, but the album can be found here: http://s81.photobucket.com/user/shuteme ... ape%20Rust