I would worry more about the batteries in the key.
I also would advise trying the remote using the trunk and passenger side recievers if it happens again.
I would also point out that two separate security systems are involved with your events. The RCL remote control is radio frequency operated to lock and unlock the doors and to command the ATA (alarm system - separate from the locking system). The alarm is not released or set with the manual key. Neither is the alarm reset with the electronic key function at the electronic ignition switch EIS. There also is a separate security system DAS that is involved with the handshake the key does with the EIS. This handshake is not dependant on the battery operated part of the key. There is a signal that is sent from the EIS to a chip embedded in the key which then resonates a return signal (powered solely by the original signal) that makes the "hand shake". Once this happens the EIS releases the lock and empowers the engine management system.
Bosch Master, ASE Master, L1
33 years MB technician