I recently had an issue with the airbag warning light staying on while driving my is250 which is out of warranty. I was disturbed to read that some is250 owners have had issues with cars out of warranty having to spend extortionate amounts of money with Lexus dealers to replace in-dash harnesses. Even if it is only a loose connection dealers are only interested in replacing the whole in-dash harness, (very profitably).
I found a simple tip on YouTube that worked for me with the same problem. Before you take the car to the dealers for a fault finding check try this simple procedure. Start the engine and run for 6 seconds, if the airbag light continues to stay on, switch the engine off and leave for 6 seconds then re-start the engine. I done this for 5 consecutive times and the light went off and has stayed off.
I can only guess that this process re-sets the engine management system relating to the SRS airbag system. If that does not work, disconnect the battery terminals for 30 seconds, re-connect and then try the same procedure, if that fails, then you will need to consult the dealer to do a diagnostic check. Hope this tip works for you.