CT6 Caddilac 2018----Car stopped. Engine off. Key fob in my pocket. I open the door to exit the car and the chime rings rapidly and won’t stop till door is closed. (Yesterday my son touched something to turn off the lights when we stopped. )
Any ideas?
your vehicle warns you when you leave something on, like the lights if you do not have it set to auto on/off.
Info about this is probably somewhere in the owners manual. Settings can be changed, and sometimes it happens unintentionally.
Me thinks that is a clue . See if he remembers what he did .
Not the dreaded owners manual!
@nybob6, I hope comment above didn’t turn you off enough that you stopped reading. The solution might be in the settings in your infotainment system. Check the settings for everything and see if they are what you want them to be. You might need that dang owners manual to figure it all out. I’ve discovered that the settings are confusing with the rental cars I’ve had in the last couple of years and the owners manual was useful.
My different brand does that if I open the door before push button off the engine.
I didn’t know how to open the gas cap on a car I rented a couple years ago. There was no owners manual or instruction sheet with the car. Embarassing.
I hear you, I try and make sure at least I know how to use the wipers and wash in a rental car!
My Corolla chimes if I open the driver’s door while the key is in the ignition. There’s an electronic sensor ( switch) as part of the ignition switch ass’y that tells the alarm system if the key is still in the lock or not. If the key is in, and the door is open, well, get ready for a couple of cords of hallelujah chorus 
This feature has saved my bacon a few times, alerting me not to lock the keys inside the car.