Argh this sucks, but I'm surprised it doesn't happen more often, especially with so many quality private collections just being kept in backyards.
I commonly find myself trying to think of a way to somewhat dissuade the potential for theft but typically they're burdensome, ugly (cable/chain internally connected to a pot, and to bench legs etc.) and not really realistic outside of maybe a tree or two. Certainly too difficult for a smaller tree which seems logically to be the most theft-prone.
The GPS device is probably the most promising sounding idea I've heard so far but the issues have been noted above.
It sucks that things like the above even need to be considered.
Unfortunately, i can't imagine cameras yield many positive results assuming the thieves are even remotely 'savvy', but are certain a good dissuasion and real-time alert.
Argh. My fingers are crossed these are found.