I did a search, but couldn't find anything on this posted previously. Until today I hadn't even unlocked the car since 16th January, so it has sat on the drive on a battery tender since then. It wouldn't open on the fob button (either fob) so had to remove the key from the fob and unlock the drivers door manually. After I did this the system make a lot of beeping noises and a warning logo was flashing in the dash. After about 15 seconds everything returned to normal. I can now lock and unlock the car with the fob. It seems like the central locking system and everything else went into some sort of shut down to the point where it wouldn't unlock. Anyone else ever had this issue after not using the car for several weeks?

Low power stand by due to being left for over a month.

Sent from my SM-G965F (S9+)

As I understand it, the keyless fobs do shut down if they aren’t used for a period of time. Look in the instruction book, I’m sure thats where I read it. 

You have to do as you say and unlock manually to get them working again.