Have 2014 Nissan Sentra. After I shut the car off if I opened the door and the headlights were on an alarm sound/ like a long beep. It doesn’t do it anymore. Anyone have any ideas why it stopped working? Thanks
You wore it out? 
Delve into the owners manual. Lots of electronics have on/off/if/only/if and only if, etc. functions that can be unintentionally changed.
I didn’t know my 1999 Honda’s doors, when opened with the remote, would relock if no door was opened within some number of seconds. A friend who was reading the manual while I drove from WI to MT and back unearthed that factoid.
Possible: If you manually turn on your headlights, the alarm will beep. If you have them on the automatic setting, they will turn themselves off, no audible alarm. As Shanonia said, the owners manual has a wealth of info in it.
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If your remote is like the one for my 2008 Nissan, you likely reprogrammed something accidentally. I frequently trigger the buttons accidentally. The “best” was opening the two front windows. During a rainstorm.