Many have had success combining multipe types of presence sensing into one device. See linked smartapp. The idea is that if one of the presence devices doesn't work, the other will. I used to do this and it worked pretty well with few issues. I have since added a keypad to arm/disarm HSM and we now use that when we leave/arrive. The keypad has been 100% reliable. It's basically the "switch" @Jost is talking about. Keypads can be had for cheap on eBay.
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