Breaking a chainstay is quite likely to be non-catastrophic failure, based on my experience. I had a Peugeot UO-8 chainstay break just behind the support. (I suspect a combination of the big crimping on both sides and many frame bending crashes. Also 22,000 miles. I never noticed it snapping. Just the bike felt a little different so I looked down and I saw the chainstay swinging in the breeze;

Ben