Over the years I have acquired a large number of Byron 433mHz wireless door chime units***. They have two sound settings - "Westminster" and "Avon" - that are selected in each bell push. The house has a front door wired bell push and a back door wireless bell push. In addition there is a "visitor" infrared beam across the drive. Several chime units are distributed about the house and property.
An Arduino acts as as a spoofing bell push on activation of either of the two bell pushes or the beam interruption. The front door is then signalled with a Westminster chime sequence repeated five times; the back door with an Avon chime repeated five times; the beam uses the Avon chime but only repeated twice.
It was a dark and stormy night - "Ding Dong" - visitor or back door? Repeats several times - and keeps going. No one in sight at either door. Nor is there a wind blown piece of debris flapping across the beam.
Eventually switch off the Arduino unit - and the chimes continue. Must be the back door bell push signalling directly to the chime units. Get wet taking it apart and remove the battery - "Ding Dong" continues unabated. Am I being pranked?
Apply logic - what has changed recently? I tidied the garage earlier - which created a tall stack of plastic boxes. One of those is crammed full of the spare chime units and accessories. Dismantled the stack - and heavenly silence. A spare bell push - coded for Avon - had a live battery. It was in a plastic bubble pack which had gradually become squashed by the compression of the box lid by the higher boxes.
It actually happened again a few years later. This time the box was stored in the garden shed - and there was another live bell push that had been overlooked. It took no time at all to guess what was happening.
***I never threw the Byron mains plug chime units away when they started to fail with discordant low volume - just bought some more off ebay. Recently I found that the X2 mains-dropping capacitor needed replacing in them - having deteriorated to a fraction of its nominal capacitance.