I agree with some of the above posts. If you drove your 'B' to the office, leaving your stuff there, the neighbor probably assumed you ran off and left it. Not saying he did the right thing by gathering it up for his own use. But pretty iffy leaving things behind like that without making it clear you were returning. Don't know whether to classify that as a thief or opportunist.
I have also seen things forgotten and left at campgrounds when people left. And I have turned them in to the office. I have left a few things myself and got too far down the road to go back to get it. Fortunately nothing expensive.