My key battery is a little low, but not so low that it doesn't open doors, etc. I've been getting a "low battery" warning on my dash about once a week (and just bought replacement batteries).
In any event, today I was able to lock my keys in the trunk. Fortunately the car door was unlocked so I could open the trunk.
I'm not sure if it's because the battery is a little low, or not, but i thought the key (located inside the trunk) would disable the trunk from locking, but apparently it doesn't. In any event, get in the habit of making sure your door is unlocked before opening the trunk if you're putting things in, if there's a chance that the key might end up inside the trunk.

 

You cannot lock the keys in the trunk of my '09, at least. My wife carries her fob in her purse. We will often throw her purse in the trunk, and if I try to lock the car using the lock button inside the door, the car will ring the long lock/unlock tone and then unlock the doors. If the purse in the trunk, the only way I can lock the car is by using my fob. I tried an additional experiment: I put my fob in the trunk, left the trunk open, locked the car using the driver's side inside door lock, and sure enough--the car unlocked itself.
If the fob battery were low, I suppose the car might not recognize a fob locked in the trunk. Did you try locking the car with your fob in the trunk?

 

didn't try to lock the car

the point being that the system, which I'm sure is designed to prevent the key from being locked in the trunk (or cabin) can and will fail under certain situations, maybe low battery.

 

I had a low battery once and tried to start the car with key fob in my pocket and it would not start. So I moved the fob to the start button and it started. That was a close call about 50 miles from home! Good to know Haroldo.