Unfortunately, by the time I was back online, that person had just left for the last time after taking all hybrids, digging up all bushes, etc. Also, though several friends in my town confirmed one person who I was not familiar with, and thought that person struck me as the only likely candidate, I also had no proof, and this person stopped being active after the incident. This was at GameFAQ, too.
It's too bad about me having to be cautious, too, since dozens of people frequently enjoyed coming to my town when the gate was open, sometimes leaving gifts, often times leaving grateful notes on my board.