In theory if the battery is low, it may not be transmitting to the receiver by the steering wheel (assumed location). Only got my car yesterday so enjoying reading and gaining knowledge.
I have one fob with a low battery. So i tried various scenarios.
1. Touched front handle with fob in pocket, car unlocks, i placed the fob in the back and closed the door, waited 30 seconds and the front door was still fine.
2. Locked everything back up, then went to my boot with fob in pocket the boot opened via the button, i placed the fob in the boot and closed it, the car alarms and wont lock the boot.
Cannot recreate your situation, still feel it could be the battery is very low.