Does anyone have a clue where this mystery bracket came from? Found it laying on the passenger side floor, so presumed it came from under the glove compartment area.

 

That doesn't look like an automotive grade part. The coating doesn't seem right. I would think it is not related to your car

 

Looks like it steadies pipes, maybe air conditioning or coolant. Or perhaps wiring harness. That coating is 100% what you see on various interior metal parts.

 

That is a clamp for the heater core tubes.

(Instrument panel removed in picture)

 

I stand corrected

 

Many thanks Nevada, this clearly explains it, as I recently had major A/C repair done to replace the evaporator core. The bracket might have been damaged by the Lexus mechanic or was installed too tight and finally broke off.