Inconsistent Mongoose find behavior

This has been puzzling me for an hour or so. I have a test like so (edited for sake of question:

var person = new Person({
  display_name: "Dom"
})

person.saveQ()
.then(function(savedPerson) {
  var incident = { 
    logger: {
      logger_id: savedPerson._id
    }
  }
  return request(app).post('/incidents')
  .send(incident)
})
.then(function(res) {
  console.log(res.body)
  should.exist(res.body._id)
})
.done(done);

I create a person, then I create an incident, setting the logger_id to that just created person's ID.

My actual request handler (edited down to smallest possible failing case), looks like so:

Person.findById(req.params.logger.logger_id, function(err, person) {
  if (!person) { 
    return next(new restify.ResourceNotFoundError("Person with id " + req.params.logger.logger_id + " not found.")) 
  }
  return next(res.json(201, person))
})

I.e. I find that recently created person by the passed in logger_id. If it doesn't exist, I throw an error.

NOW... this works normally and as expected 9/10 times. But occasionally it will fail with an error saying that person could not be found. Even though they were just created, and I have verified that it existed (just printing directly after save callback).

Is there anything obviously wrong? Is it perhaps my test harness code where I create my test DB, and clear it after every test?