UPDATE: This is the last line I'll give them to try and fix the problem. We agreed to get Lexus engineering involved at this point. With all the logs and data they went through, they are going to swap out the ECU module controlling the brake functions with one off of a new RX, and put my original module into the donor RX. In theory, as an Electrical Engineer myself, it makes sense if we are chasing a weird intermittent problem. I would hope that the issue follows the original module and that Lexus can isolate it and of course replace it with a new module from inventory and then send that problematic ECU to engineering for a troubleshooting tear down. Since the problem is intermittent and no rhyme/reason when it comes on, I've asked them to keep the vehicle for several days of testing. I sure hope they see an error in one of the RX's.

Another update to come.

 

Got a call last Friday that they still weren’t able to get any errors in my vehicle with the new ECU, or the new vehicle with my old ECU after driving both for 300 some miles. I asked to keep through the weekend and told them that the 2 times I personally saw the error and light on was with the car sitting idle and my foot on the brake and then again just parked in my garage. Well, got a call yesterday that they finally seen the error in the new vehicle with my old ECU about 1pm with it sitting parked in idle and visibly seen the brake lamp on after they shut the car off. That’s good news hopefully! They kept the new ECU in my vehicle and returned it to me today and assume are sending in that faulty ECU to Lexus engineering and putting in a new ECU in that vehicle. I really hope this is the cure, because I ain’t settling for anything less than an exact replacement brand spanking new vehicle from Lex if it comes back.

 

I applause you for your patience and hope that swapped ECU will take care of it.

I once dealt with a '17 NX Fsport with the trouble navigation/backup camera stuck for roughly 6 months, gave up and get rid of it after 1 year of ownership