It doesn’t integrate with SmartThings, but my brother has been using Tagg (now owned by Whistle) for several years with his dogs, including 1 who is an escape artist, and he’s been really happy with it. There is a monthly fee, but it has a GPS tracker so you don’t just know that the dog got out of the yard, you know where the dog went. You get a phone and email alert if the dog leaves the home zone. He’s recovered his dog at least three times since using it, so he says it’s well worth the expense. 
It’s fully waterproof and very durable. (It has an IFTTT channel, but just for the fitness tracker, not the GPS info, so it’s not that useful yet.)
General Issues
There are some other issues in device selection if you’re looking at something to connect to SmartThings.
The first is whether your dog plays with other dogs. This makes a big difference because dogs do chew on each other’s collars. So if you have two dogs, or you regularly take your dog to the dog park, another dog might chew on the device you attach to the collar. The whistle seems to hold up well in this situation. A lot of I beacons wouldn’t.
The second is whether it’s waterproof. There are a number of devices that are, but make sure you check.
The third is whether it can send a text. (Whistle can.) If it does, you can send a text to IFTTT and get indirect integration with SmartThings that way.
speaking just for myself, I’d choose a device that can tell me where the dog is after he’s left home, not just one that tells me he left. But everybody’s needs are different. 