W124 dieseled for maybe 15 sec after shutoff

[jbach36]

I'd previously seen posts of people's cars running after they turned the key off, or some that won't shut off at all. It's a bit nippy outside today, maybe 45 degrees. I started the car up cold, backed out of the garage just 25 feet to put it in the driveway while I clean out my garage. Turned off the key, and it continued to run for about 15 seconds, first time ever it's done that.

I figure it might be a combo of the chilly weather and that the car was only started for a 10 second drive.

Am I right into thinking this is a one-time-thing, or is this a foreshadow of bigger problems later?

Thanks,

Jeff 1991 300d, 102k