There was a big theft at a nursery in SoCal, and I'm not sure to what extent they ever recovered. The thieves drove up behind the nursery in a truck and cut through a chain link fence. I heard they took quite a number of trees - including some that were chained down and retailed for $1000's.
The nursery installed a barred security fence that they now lock at night. However this also reduced their bonsai area in size. Thieves suck.
Though sometimes the thieves are bonsai enthusiasts, there are times they are not - and are just attracted to the trees as something they perceive as being valuable. In one case, at an exhibition in Florida, they caught a thief on security camera as he walked off with a large tree when the exhibition was being taken down. The Police were able to track the suspect to his home and recover the tree... and the individual had no experience in bonsai.