The third wire in the detector is often yellow...it is the lead that should be run between the two detectors so that if one goes off, it triggers the other one to sound as well. White and black are neutral and power, so it sounds like they expected the red to be that interconnecting lead. That's the normal thing, but yours could be different. With the power off and the detectors disconnected, if you turn it back on and measure from the red to ground or neutral, it should read zero. This assumes you feel safe about using a multimeter and won't hurt yourself. What brand and model?