mcomer

The missing key chime only sounds three times. Recent events of leaving a keyless
car running has prompted calls to change to some greater notification but for now
you just get three relatively quiet chimes.

dchar

Option 1:
If in store for less than 10 minutes, you can use the Enform Remote to leave the car running. If you use the phone app, you can see a countdown. I think the app allows you to restart the car up to 3 times until you have to start it from inside again. So technically you have 30 mins, but you'll have to keep checking to make sure it's still running. As long as she doesn't open the door, the car will continue to run.

Option 2:
Leave car running after parking, take fob with you, lock door from switch on driver's door, and close door. If the NX is like other Lexus models, it'll beep 3 times and continue running (I don't think it beeps any more than that). Then unlock the door with the metal key when you return.

Didn't realize how old this thread was...so you probably already solved your problem.