Why is placing the Key near metal a problem?

In the Owner Manual it says "Do not attach metallic or magnetic materials to the keys or place the keys close to such materials."
Since the Key has a battery it is clearly an active device. What is the reason for this warning, particularly with respect to metallic materials? A technical explanation would be appreciated.

 

Placing the key in a metal foil is exactly the wrong thing to do if "storing near metal might damage the key". Such storage may not be a problem as described below.

In a RFID system I believe the ID card is a passive device that reacts to the RF from the stationary transmitter/receiver in a manner that provides the code contained in the ID card (or my cat's ID chip). That wouldn't require a transmitter in the vehicle Key except for the usual lock/unlock of control when you press the button. If that's the way the Prius Key works being near metal when using it might be a concern per your Faraday effect comment. However, that doesn't doesn't tell us why storing the Key near metal could "damage" the Key.

Other comments on this theory would be appreciated.

 

After more investigation it appears that the RFID system in the Prius Smart Key is active rather than passive. That is, it uses power from its battery to respond to requests from the vehicle for ID information. That makes sense in conjunction with the warning about not keeping the Key close to electronic devices. I can see where the Key might respond to "interference" signals "thinking" they were requests for ID information hence run down the Key battery. I don't see where having metal nearby would cause any problems and equates to storing a Key in the vehicle wrapped in aluminum foil. What it does say is that leaving the Key in the car for long periods of time (say in your locked garage as I have been doing) will discharge the Key battery because the vehicle will keep requesting ID information from the Key. It is interesting that the manual doesn't say anything about leaving the Key in the vehicle except with respect to theft protection. Is this actually a situation that will discharge the Key battery?

I have also read that leaving the Key in the vehicle will run down the 12 volt battery. I don't believe that should be much of a problem considering the very little amount of power required for the ID communication compared to the power to run all the Key communication oscillators at various places for outside ID (and inside?) detection which are running along with the computer to control all this even if the Key is nowhere nearby.