Need to find the specific term used for someone who has something unexpected and bad happen to them and who then alerts others to be on the look out

I am trying to respond to a co-worker who sent an email to all staff to warn us to lock our cars and close windows tightly because her car got broken into the night before and she had left her windows down a crack. I wanted to send an email to thank her for the warning but sorry she had to be the “____”. “Guinea pig”, “first victim”, “example”, don’t feel right in this context. Any other suggestions? I thought there was a word for someone who has something happen to them who then alerts others to be on the look out.