So there are two that I know for sure are in so-cal and that is the the California mantis, S. Californica, which is a native species and, surprise surprise, the Chinese mantis which is of course naturalized all over the US. I know this because my mother in law saw a female S. Californica in her backyard a few days ago and I just caught a little male chinese like an hour ago. My first wild caught mantis btw.
So in answer to your question, more than likely it wasn't a stowaway. Mantises are pretty delicate. But if it was a Chinese it's possible. A S. Californica though is native so it's not a stowaway.