On replacing smoke detector batteries

We had a dinner guest Saturday night and during dinner one of our detectors (not near the kitchen) started going off. Wow that was annoying! We have five Kidde i4618 and one carbon monoxide detector. Kathee dragged the ladder in from the garage and waved her hand around what we thought was the offending detector - no luck. I suggested that she vacuum inside of the detector. No help. I ran out to the local convenience store to buy one 9 v battery (wow - $ 5.69. The Battery Plus next door had just closed). Kathee replaced that battery. No change. I looked at the owner's manual on-line and suggested that we should replace the other's batteries too. At that juncture I thought we had 4 total not five. Our dinner guest and I drove back to the convenience store to buy 3 more 9 v batteries. We we returned, Kathee informed us that we had 5 detectors not 4. So back to the convenience store for the fifth one. After replacing all five batteries, we found out that it was the carbon monoxide detector, not the smoke detector. It was in close proximity to the first detector and with the noise we were not able to discern that was going off not the smoke detector. So 6 batteries and an hour and a half latter we finally got all resolved. Note to self ... replace these batteries annually and buy in bulk - I could have bought 8 Duracell Coppertop Batteries's for under $ 20 from Amazon. Instead I paid $ 5.69 each at the gas station convenience store.