Back in June, a user SBCYAHOO posted this subject regarding their 06
Elantra. I've posted about this several times before - so you aren't
alone.
My 06 Sonata LX has the same issue, yet, I can never get this to
duplicate at the dealer, so I get no satisfactory results or repairs.
I have shown them a short video taken from my digital camera, and they
still wouldn't do anything. At this point, I have given up, if it;s
acting up, I'll just turn the car off and restart it.

I notice it the most under three conditions:

1) A really hot New England day, windows are rolled up, the heat
inside the cabin causes the passenger buckle light to flash.
2) My passenger was just sitting in the running car, left and came
back a short while later, and the air bag light would stay lit
(indicating air bag is off).
3) New passenger enters the car and air bag light stays lit
(indicating air bag is off).

Items 2 and 3 are intermittant, of course, when they occur, its never
convenient to head to the dealer. Item 1 ALWAYS happens when the sun
has been directly beating down on the car.

I understand the dealer just can't replace it base on my word. So,
you're not alone with this problem.

-Matt B

 

"I have shown them a short video taken from my digital camera, and they
still wouldn't do anything. At this point, I have given up, if it;s
acting up, I'll just turn the car off and restart it."

This is the troubling part of your post. This is obviously a safety
issue. Furthermore, it's almost a lock that the issue is in the passenger
seat bottom. What was the dealers reason for not doing anything when you
showed them the video? While I agree they cannot perform proper
diagnostics if the problem won't occur, the video blows "unable to
duplicate" out of the water. If you haven't done so already, call
Hyundai's customer assistance number (800-533-6161).