Any key that doesn't need pressing to access the car is at risk. The solution is simple.
1. Buy tin of cookies (in an actual metal tin).
2. Eat said cookies!
3. Brush out crumbs from tin and place it where you normally keep your keys
4. Place keys (including spare) in tin
5. Keep lid closed
If you want to test the solution, take the tin containing the keys to the car door and try to get in. The car should refuse as it cannot see the RF signal from the key.
Don't bother with all the fancy gismos online called Faraday cages etc etc. Cookies are tastier!