Its a less racial way of saying there are black people somewhere on your property. We say it when blacks show up at our haunted house. The blacks are usualy the ones that hit our monsters and get all kung foo on that ass when they get scared. So when they show up, a call goes out over the radio that "there's ninjas on the lawn"
I have also used it when there are black ppl around my store. Like when they hang out in the parking lot doing nothing.
I saw a group of black people headed for the entrance to our haunted house, so I alerted the managers that there's ninjas on the lawn.