The red wire is the trigger that causes the other alarms to turn on...it should NOT have 120vac on it! Someone has potentially miswired it, and if in fact it does go to the other alarms in the house, they are suspect.
If I had to make a guess, that cable probably goes through an electrical box somewhere, and somebody connected the red wire to a switch leg of something, thinking that all of the red wires should be connected together. It might be a pain to find the problem. Another possibility (and there are lots of them), is that the insulation got damaged, and it is shorted to a hot wire somewhere. Finding that could be a pain as well. If someone did not use nailing plates to protect the wire, and they hammered in a nail to hang a picture, that nail could have shorted things together.
You could leave the red wire disconnected and get a new unit to work until you can find out where that red wire is getting the improper voltage. It won't trigger the other alarms in the house, but you may not care.
If you're interested in a better alarm, you might consider the Nest Protect unit. I put some in about a year ago and then installed some at my mother's house. Those do NOT use the red wire, and talk to each other via a closed, wireless connection. They also 'talk' on your WiFi network, and you can monitor them from remote...if one goes off, it will send you an e-mail. They are a combined smoke and CO detector, last 10-years, which is a long time for a CO detector. FWIW, all of the companies recommend you change your detector at 10-year intervals, even if they self-test okay. Those units are dual-mode, which means that they detect both fast/hot fires, and slow smoldering ones. Most units only detect one type of fire, and can allow things to progress further if the fire happens to be the opposite of what it's best at detecting. Eventually, either type of fire will trigger either type, but there may be a delay. These detect both, so get a faster response all of the time.
