Had this message randomly come up on the dash when using my original smart key and dash message informed me that I needed to place it near the receiver (near left hand side of the steering column). Didn't make a jot of difference. So, resorted to using the spare smart key. That worked fine for a few weeks, until today when I had the same message with the second smart key

So went back and tried the original smart key, dead as a dead thing that just died. Resolved by replacing batteries (CR2032) in both smart keys and now all good. The L322 used to warn me when the smart key battery was low and according to the L405 manual it should do the same.....but it didn't

Car is MY18+ (latest shape) and the way around getting the car to start is to use the remote start app on the I-phone, as long as at least one of the smart keys still has enough battery power to open the car.

Another fullfat lesson learned  SDV8 AB MY 2019
SDV8 VSE MY 2015 - Gone
5.0 S/C AB MY 2010 - Gone
4.2 S/C VSE MY 2006 - Gone

I was told by a very experienced LR Technician that the newer square 2018 onwards fob batteries last around 18 months on average, much less than the earlier fobs.

I too had the same experience as you yesterday on my 2019. Replaced the fob battery and everything is fine again. Keep a few CR2032’s in the cupboard and remember to replace them every year or so. MY23 D350 HSE
On my 7th RR plus various other JLR vehicles

Sorry, only just seen your post Phil. Thanks for that, useful to know and I've done the same and stocked up on some 2032's!  SDV8 AB MY 2019
SDV8 VSE MY 2015 - Gone
5.0 S/C AB MY 2010 - Gone
4.2 S/C VSE MY 2006 - Gone

I can confirm that, although in the manual it is written that "the instrument panel will display Smart Key battery Low", it didn't, but I got the "Smart Key Not Recognized" message all week... Replaced battery (less than 12 months old) and all is good now...

Also, inside the Smart Key, there is a yellow sticker saying "do not store without battery"...

Finally, in the manual, they also say "replace the batteries in all of the smart keys simultaneously"...

Had the same happen to mine after 6 months, couldn't believe it was the batteries on both keys, rang my local dealer and was told that the keys have the batteries installed at manufacture and can sit on the shelf for some time before being coded to the car.