LOL! Would love to know the answer here as mine does the exact same thing and only started after I picked it up from the dealer, upon it's recent service visit.
Triple check the engine hatch cover. Mine double clicks almost. I had the latches replaced under warranty and then realized I wasn't closing the hatch hard enough as the new latches were tighter.
It's definitely not something open. I remember the sound the car used to make when something was left open, but it was never three audible, independent beeps like this.
Before I comment keep in mind I am a new Mclaren owner! Just a total guess. If the dealer reprogrammed the computer for some reason,
it might of gone back to default settings and turned on a warning chime that was previously not on. Just a thought!
It’s 3 fast beeps and turn signal lights blinking hectic. Tried to close everything properly several times. Shouldn’t it be just one long beeeeeep if something is not closed?