I was under the impression that the car did not have a "proximity" sensor for the key, since its not a "smart" key.
That's not true. It does have one, but it's in the center dash and only has a range of about a yard at best. It's part of the security system, which essentially does an RFID check on the key. If you have other RFID based fobs on your keychain within the same frequencies, they can sometimes block your key from replying, or make the car get a wrong number, and it won't start up, giving you a key symbol in the dash display. (Ask how I know...)
It could be you have a failing or incorrectly installed motion sensor if you have the OEM security system. They can fail randomly based on temperature and moisture settings, especially when installed upside down (which smart often did in earlier models).
My 2008 had this issue several times, and I went through 3 sensors before finding one that was reasonable. Even then, it still occasionally fails if conditions are just right: If there is moisture in the car and it goes below freezing, a light frost shows up on the sensor as motion and it goes off. Nights where it's around 0°C, or transitioning, I have to hit the "tow hook" button in the cabin to disable the motion and tilt sensor or risk having to go out in the cold to reset the damn thing as it keeps going off.
