2009 Lexus ES350 intermittent airbag warning light.

The whole idea of stored diagnostic fault codes is to aid in diagnosing intermittent problems such as this. Finding no codes suggests they're being erased due to a power supply problem, meaning loss of 12 volts to one of the terminals on the computer. There will always be two 12 volts supplies. One is needed to power the warning light when the other one is missing. On most cars, either circuit will keep the fault codes in memory, but if the constant "memory" circuit is dead, the codes will erase when you turn off the ignition switch which turns off the "switched" 12 volt supply. Some car brands retain the fault codes in memory even if the computer is completely unplugged. Those codes can only be erased by connecting a scanner to erase them electronically.

I would start by looking for the two fuses for the air bag computer. Be sure they're tight in their sockets. Try wiggling them while watching the air bag light and with the ignition switch turned on. If the light turns on while you're driving, make you way to the dealer or a mechanic with a scanner that can access the air bag computer, but don't turn the engine off. The code(s) should stay in memory as long as the ignition switch stays on.