I had this issue multiple times last November and worked with support on it. They verified that it was happening, but never found a reason or a fix. This was a standard lock using standard code, nothing custom. After two weeks, for safety reasons, we switched the lock to a different non-smartThings system and had no more problems with it.
Several other people have occasionally reported garage doors opening when they weren’t supposed to.
At my house, we would be more likely to notice the problem, because I spend most of my life in one of two rooms and one of those is the one with the lock. Also, my service dog is trained to go to the door when it unlocks as he is the one who physically opens the door.
For this reason, I’m pretty sure that the unlocking never happened between November 2014 and October 2015, but did happen multiple times after that.
I would say now that a smartthings – connected lock/garage door is unlikely to open itself, but judging just from forum reports is more likely to do so than one controlled by Insteon or HomeKit or Control4. I don’t know about other controllers. I can’t imagine Yale or Schlage would even manufacture Zwave locks if this were a common problem.