I was driving around town, LDA was not activated, I wasn't using cruise control, weather was good and no one else was on the road. Suddenly, there were two quick 'beeps', I thought I saw something flash yellow on the Information Center, but it and the beeps were gone before I could look down. No warning lights are on and no messages in the Vehicle History. Any idea what that possibly could have been???

I re-drove the same route. At a point where the road takes almost a 90 degree turn there is a car parked on the shoulder. As I approached, before sharply turning to the left, Lexus thought I was going to hit it and warned me to BRAKE!, along with the warning beeps.

Good idea to reproduce the warning message to troubleshoot!

I have a similar (no sound) warning, just the flashing indicator on the driver's side mirror as if there was a car approaching on my left lane, when I did a close to 90 deg left turn in an indoor parking garage. The NX was close enough to the wall when turning which triggered the yellow flashing indicator, no beeping because I did not turn the left turn signal on.

Yeah, the pre-collision braking system does sometimes false alert...there's one sign post in the median on a road I drive with a slight curve that occasionally will trigger mine.