Normally people post a problem and the community chimes in.
This time, I wanted to share both the problem and a fix in case someone else has this issue. Consider it my small token of appreciation for all the times NAM helped me.
I'm posting a lot of details here as I will likely be too busy during the week to reply to any questions.
I'll post some pics later if I have time.
Hope it helps!

Background
About a month prior, I had water leak into the battery cubby in the trunk due to heavy rains. (I think it was a due to rear sunroof drain being clogged. I cleared it afterwards.)
The water sat in the trunk a few days before I noticed it. I removed the battery and wiped down the condensation then kept the felt/foam lined battery cubby cover opened to let the area dry out.
Car was starting fine throughout this time until a few days ago.

Symptom:
Turn key to on. Instrument panel lights turn on as normal.
Turn key to start. All lights turn off. Click Click Click sound from front of the car.

Fix:
Clean all power and ground cable ends and the mating surfaces on the car chassis to which these cables connect.
By clean, I mean remove the oxidation to expose shiny metal using a file (sandpaper might also work) on both sides of the cable ends and anything it will come into contact. There may be gentler ways to do this.
This is important as some of the surfaces may look "ok" but can have a film of contaminant that limits the electrical current.

List of cables/ends/chassis points. Most are attached with a 13mm nut. 19mm socket is needed for positive post.
1. Under the hood, passenger side.
a. Black ground cable (passenger side). connects to engine and chassis.
2. Under the hood, drivers side.
a. Positive terminal post (located under a 2"x2" square plastic cap, attached to the side of the air filter box); the Post is a 19mm "nut". Remove the nut and clean the bottom of it.
b. Metal strip inside the terminal box.
c. 2 positive cable ends attached to the post. Clean both sides of each end.
3. Battery cubby, trunk. Remove the battery to gain access to all power cables. Open the power junction box, located on passenger side of battery box, and remove the 3 13mm nuts.
a. Battery - positive and negative posts. Use a wire brush to clean it.
b. Black ground cable, drivers side
c. Red cable 1 - connects to positive battery post
d. Red cable 2 - connects to red cable 1's battery terminal 1 by a 13mm nut. The other end connects to the
e. Red cable 3 - connects to power junction box
f. 200AMP fuse.
g. Metal bridge/connector, about 1.5 inch long.

I'm not sure which of these were bad as I cleaned and reattached them all before trying to start the car (successfully). This took about 30 minutes.
My guess would be one of the cables/ends in the battery cubby was the culprit.

Here are the other things I tried/noticed which made no difference, except for #4. Most of these took a few minutes and were free.

1. Tried to start car using my spare key
2. Charged the battery at local auto store - Store said battery health was GOOD. Voltage measure 11.8V. Purchased in 2015.
3. Jump start the car with cables attached to battery.
4. Jump start the car with cables detached from battery - car started a few times when the donor car was a SUV. Never worked when car was an old lexus sedan.
5. Checked the fuses related to the engine (marked by an engine with lightning bolt) in the fuse box in engine bay.
6. Shuffled the green relays in the fuse box above.
7. Cleaned the ignition switch; took about 1 hour to disassemble/reassemble; be sure to remove the lower part of dash (held on by two clips located at the upper left and hand corner). This will save tons of time and prevent damage to interior.