the SmartThings presence sensor is goofy. There are many many weird things you can see with it. So if you are seeing wonkiness, my own first guess would be that it’s because of that device specifically.
What does “Away” mean?
You know how SmartThings decides that particular device is “away,” right? It’s set to check in every 30 seconds. That is not configurable. It’s always once every 30 seconds.
What is configurable is the number of missed check ins the cloud will allow before it arbitrarily defines the Presence sensor as “away.” For example, if you have a presence sensor which keeps bouncing back-and-forth between home and away usually the first thing support will do is increase the number of allowed misses. So this is not a zigbee activity – – it’s all software in the cloud. The check-in is zigbee but not how the account responds to it.
Keep the Battery fresh so the signal is consistent
Low battery strength can absolutely drive the system bonkers. One of the things that it does is throw off the 30 second timer. So you should Never use a rechargeable battery in one of these devices. And change out the battery whenever it gets to 50%.
Repeaters and the presence sensor
The presence sensor is most accurate if it never uses a repeater. I’m not saying it can’t use a repeater, because it can… But again there’s a tendency to throw off the 30 second timer. Indirectly, anyway. I don’t want to go into all of the technical details but it just works best if the presence sensor spends its time at home about 10 m from the hub (and from any Wi-Fi transmitter) and then gets picked up and taken out of range within a minute or so. If it’s more than one hop from the hub while it’s also “home” Peculiar things can happen. Although it may be fine if you leave it in your car and it’s always connecting to the same repeater in the garage.
Where things will get flaky is if it moves around your house at a distance of more than one hop from the hub but you still want it to always show as home. It will work OK for some people, but it definitely won’t for others. This is one of those “all home automation is local” situations.
The time it takes to find a different repeater and get the message through may cause it to be tagged as away even though it’s really there. Other zigbee devices don’t have the same issue because the cloud isn’t monitoring their check ins in the same way. An occasional delay for them is no big deal.
More discussions
There are lots of threads in the forum on the presence sensor but the short answer is it works great for some people and not at all for others. It is definitely subject to Wi-Fi interference. It works best with the battery 50% or above. And the time out parameter is configurable but only in terms of the number of missed 30 second check ins allowed for the grace period.
Oh, and if you’re using it in a car, adding one zigbee repeater in the garage may help a lot. Notice that in this last case, though, the presence sensor only has two locations from the hub’s point of view: in the garage or away. So it only talks to one repeater on the network, the one in the garage.
Short Answer: Give it a permanent home
So the short answer is if you’re going to use the smartthings zigbee presence sensor, use it as a “crossed the geo-fence” indicator and pick one place in your home where it will always live when you’re not away. 
More on the presence sensor here: