Been lurking here for a long time but only just registered.
I am from South Africa and have a 2013 GS350 EX which has intermittently given me navigation problems where I would, suddenly out of the blue according to the nav, be travelling over dams and on no roads and could never replicate this when I took it in for the Lexus garage to check and no faults were ever logged in the car computer, until today! The technician saw first hand exactly what I had been complaining of since getting the car and he said that the navigation receiver was faulty and would need to be replaced. Luckily my warranty ends next month so the new parts were ordered under warranty and the car needs to be booked in for 2 days for the repair which includes removing the dash. Petrified that I'm going to have rattles after I get the car back though!
If anyone has experienced a similar occurrence, this is your issue and it WON'T log as a fault in the car's computer.

 

My GS navigation exhibited the same fault. The antenna is located within the dash on the passenger's side and I had added a dash camera with a windscreen mount.in close proximity. Could this or similar be your problem?

 

Hi Mike
No I don't have a dash cam.