Is hearing people that aren't there at night a sign of pre schizophrenia?

Could be. As drugs and other disorders can cause such symptoms, would have to rule those out before considering schizophrenia. Of course the age is an important factor here.
Sometimes. Hearing people at night that aren't there is can occur when first going to sleep at night or just waking up. In most cases it doesn't indicate any disorder. There is no evidence that this is pre-schizophrenia although people who are developing schizophrenia can have the same experience. In schizophrenia there commonly is progression to extensive hearing and seeing things" believed" to be real.