Button ID help...is this stock or added by someone?

Judging from it’s out-of-the-way position, I would venture a guess that its part of an after market alarm system.? To say the least, from an after market accessory of some sort. Most certainly it is not a stock configuration.

 

Agreed. Unfortunately, they violated the #1 rule in after-market installation which is: "drill no holes; a good install can be returned to 100% OEM." Considering the number of unused blanks on the dash, it's surprising.

 

Oh darn. My hopes for an ejection seat dashed!

 

Some dealers put the KARR system on early gen 3's to protect theft off the lot. It has a few more bells and whistles than the OEM alarm: Motion detection with "warn away", shatter sensors for glass breakage, rolling codes and valet mode. Here's a writeup on how it works: (If it ain't broke, don't fix it!)
Re-programming KARR alarm
Make sure Karr is disarmed (no LED lights)
Turn Prius on
Press and release Karr button 3 times
Short LED light with 1 short horn
Before 3 seconds, turn Prius off
Horn will give a short-long chirp
Within 3 seconds, turn Prius back on (now in programming mode)
Horn will give a long-short chirp
Press Karr button one time per option (there are 4 options) the Karr LED will flash the number of times per option you are on.

Option 1 (one flash): wait ½ second and horn should chirp once (auto-lock with ignition on)
Option 2 (two flashes): wait ½ second and horn should chirp once (auto unlock with ignition off)
Option 3 (three flashes): wait ½ second and horn should chirp twice (passive arming and lock without lock)
Option 4 (four flashes): wait ½ second and horn should chirp twice (active arming)

Turn Prius off to exit programming.
Get out and arm…LED should flash.
Wait a bit and then attempt to disarm with fob.
Valet mode – car on, press and hold for 5 seconds until solid blue LED
Valet mode off – within 3 seconds of Prius on, press and hold until LED off

 

HA! Couldn't resit!