Congratulations on figuring it out! It’s possible that it was a coincidence, but not likely.
You know how you can have Wi-Fi dead spots in the Home?
Exactly the same thing can happen with either zigbee or zwave. It just means the hub is having trouble communicating with the device at that specific location. The arrival sensor is zigbee, so likely the first place that she put the purse is just in a zigbee dead spot in your home.
There are a number of different things than can cause that.
One) local Wi-Fi interference. Wi-Fi is a much stronger signal Than zigbee, but in the same band. So when ia zigbee device is too close to a Wi-Fi device, the zigbee signal can get drowned out by the Wi-Fi signal. For example, if you have a Wi-Fi booster in your home and you put the arrival sensor down next to it, it’s very likely that the sensor will then be reported as “away.” Sometimes just moving it three or 4 feet is enough to make a difference.
I have mentioned before that we have a Wi-Fi booster in our house. If I put it on the west wall of one room, all the smartthings-controlled zigbee devices to the west of it drop off the network. If I just move it to the north wall instead, everything works fine.
architectural barriers. This can be anything from a refrigerator to insulation in a particular wall to metallic wallpaper. In this case, it’s likely that the barrier is blocking all RF signals, so the same section will probably be a wifI dead spot as well. Again, sometimes just moving the arrival sensor a few feet will bring it to a clear area.
weak mesh. a zigbee device can transmit about 40 feet. So if you have a device which is more than 40 feet from the hub, it needs to pass its message to another zigbee device which can “repeat” the zigbee signal, essentially passing it along so it can reach the hub.
Zigbee repeats only for zigbee. Zwave repeats only for Z wave. Battery powered devices do not repeat.
Almost all mains-powered mesh devices will also act as repeaters with the exception of smoke detectors. Typical repeaters are light switches, pocket sockets, and in wall relays. Again remembering that the arrival sensor needs to be able to reach a zigbee device, not a zwave device.
So it’s entirely possible that the arrival sensor is in range in one part of the house because there is a reachable zigbee repeater (most commonly the SmartThings-branded pocket socket), but might be out of range in another part of the house because there was no reachable repeater in that zone.
When you combine two and three, you can get a house with multiple dead zones pretty easily.
There are a couple of ways to address it. The easiest, since you do have a known place where it works, is just to always put the arrival sensor in that area.
The other alternative is to try to strengthen the mesh by adding more repeaters to cover the place where she wanted to put the purse the first time.
Both approaches are perfectly reasonable, it just depends on what your priorities are.