With Amazon Basics 9V batteries sitting at about $1.50 a piece, I've started the practice of just replacing all the smoke detector batteries when one of them starts getting low. When I get to the end of that supply, I'm going to switch to rechargeables and just recharge them all before they start chirping. One night of bad sleep from the option of laying awake, anticipating the next chirp, or going around the house changing batteries in the middle of the night isn't worth it.

The first time I had this happen I didn't know what the sound was and it took me weeks to work out because it's so hard to locate where it comes from.