WOW - so many great responses - I'm glad I asked the question. I think I'll start dropping the small chicken door at night so they are confined - I was waiting till they started laying eggs to totally lock them in but think now I probably shouldn't wait. The reason I asked the question in the first place is that I've read how so many of you had what you thought were predator-proof coops, with chicks all locked in, and suddenly found them dead and are trying to figure out what happened to them. Got me to thinking that even if I locked them in, obviously nothing is totally predator-proof, and as a last means of defense, would my dog or I hear anything if it got in and was killing them. I like the baby monitor idea, not as a defense because if their lives depended on me waking up and running out there to chase off something - they're GONERS. But I think it would help me sleep better knowing that if they were alerting me to a problem I would at least know what the problem was, where my "weak link" in my predator-proof system was, so that I could fix it. My three children are grown and gone and I never used a baby monitor with them but I always knew I was such a light sleeper, if I even sleep at all, that I would hear them at the first "whaa". Part of the question was that I didn't know if I was lying awake at night listening for something that no one has ever heard to begin with i.e. chickens yelling for their lives.
Keep up the great responses - they provide information that I've never found anywhere else and I'm sure others find it helpful and informative. Funny, I worried less at night about my real babies than I do about my chicks. Each night I'm imagining I hear something out there, each morning I go out and do a head count.