Humidity and heat
I've had two MBPs inflate the batteries on me, so this goes back a lot further (and is more endemic) than the recall suggests.
The first time, I noticed that the keyboard was failing and the entire MBP was bulging at the bottom. When I took it to a local repair-agent (not an apple certified place), he cheerfully started unscrewing one of those damn star-screws Apple uses. The screw was about 1/2 unscrewed when it just popped from the pressure inside, bouncing a meter off the bench with a ping.
From that point on, the repair man treated it much like you would unexploded ordnance; he was very carefully removing the remaining screws, with his face away from any potential blast . The battery was about 2x the width it normally should be, and was bulging at the seams as whatever toxic-hellstew they use to drive it was struggling to get out.
This happened with a 2011 MBP and with a 2016 MBP (I sadly make apps for iOS, so... no choice). I live in a hot and humid country, so that's likely a contributing factor. If there's no bulging, or pressure on the keyboard, you're likely safe for now. If you start seeing any curvature on the bottom, run don't walk, to the nearest Apple facility.