Well, it is certainly not proper, but there is no "law" against it.  If the case goes to trial, you could allege that the officer who sat with the D.A. has a bias as evidenced by this unusual behavior--but it's probably not going to change the case one way or the other. Next time I would suggest you or your attorney request that the bailiff ask the officer to sit "behind the bar" since he is not a party.  The bailiff can refuse or follow your request and tell the officer to go sit in the back.  The bailiff and the judge run the courtroom and they can let anyone sit wherever they want.