This is my first issue I've had with my car, it has 82,000 miles on it and is 100% stock with the exception of a battery that was replaced last year. I was going down the freeway at about 75 when my car suddenly completely stalled for about 1.5 seconds before starting right back up. I pulled off at the next exit and while at the frontage road stoplight my headlights dimmed, then seemed to surge brightly before going out entirely. They were out for about a second before coming back on. My radio head unit (No GPS or infotainment screen) also seemed to be flashing itself on and off while all this was happening. The car then stalled and restarted twice within about 5 seconds. After all this, I managed to drive home without any further issues. Today, I had a diagnostic done on the car. The battery and alternator are functioning normally and there are no check engine/battery/service lights on or codes being thrown. What could be the cause behind this and how bad of a dent am I looking at?

Thanks in advance!

 

Was a BMW/MINI compatible code scanner used to read out codes out of all ECUs? I'd be very (!) surprised if an R56 has not thrown a single code in eight years/82k miles.

Was the battery checked under (heavy) load?

When the lights surged brightly at the stop sign: was the engine idling stable?

Did the engine come to a complete stop when it stalled twice at the stop sign? And if so: did it re-start (both times?) by itself without you touching the start button?
FWIW: my sister once managed to stall my R56 with an automatic gear box and it did not re-start by itself.

Either way: It sounds, as if the car experienced a voltage spike and drop. Assuming that the code scanner did communicate with all ECUs and the alternator is indeed fine, I'd concentrate on the battery first and also check for signs of wire damage.