Smart Key batteries

[ktclm06]

Hi all.
Hopefully someone may be able to give me some direction on this new one to me!.
2009 XF, 108k on the clock.

# All of a sudden BOTH smart keys (normal & spare) when used bring up the "key battery low" message on the dashboard
# Batteries duly replaced in both keys
# The message persists despite the twin battery changes and I've even changed the main key twice just to be sure the batteries weren't out of date

Now the only thing of difference I have noticed, is that when locking the car, she locks, mirrors retract all as normal BUT within 3-5 seconds (while walking away) ) I get a "double bleep" which has never occurred previously. Now to my logic this has to be part of an issue?.

Thoughts, advice, guidance, prayers all appreciated in advance.
Keith T.