What is this?

Rogues are very keen in their profession, and know already much more than we can teach them respecting their several kinds of roguery.

— Rudimentary Treatise on the Construction of Locks, Charles Tomlinson (ed.), 1853.​

Now as then, the black hats are far ahead of us. Here is a sampling of articles:

• “Keeping Your Car Safe From Electronic Thieves,” New York Times, April 15, 2015.
• “Watch This Wireless Hack Pop a Car’s Locks in Minutes,” Wired, August 2014.
• “Car Theft by Antenna,” MIT Technology Review, January, 2011.

To make attacks like these more difficult, you can activate the Electronic Key Battery-Saving Function (see the Owner’s Manual) on any electronic keys that aren’t in regular use, and as noted in the first article, keep the other electronic keys in a Faraday cage.

 

There's a setting (I think I saw it in Carista) to allow/disallow starting the vehicle when the key is detected in the hatch area. By default it's allowed.

My guess is that by removing the No.1 sensor it thought the key was in the hatch and allowed normal functioning. If you turn this off AND remove the No.1 sensor, I imagine you might find the vehicle refuses to turn on using the smart key.