If you don't take the batteries out of the fob, the car goes nuts.

I went on a long trip and took my spare fob along. Even wrapped in tin foil the car would sometimes see the fob and refuse to lock the doors. And this also leaves the trunk "unlocked".

Taking the batteries out fixed the problem.

I took the fob along as a precaution. I was out boating a lot. If I got locked out, I could always call a tow truck to open my door, or in the worst case I could break a back side window.