Well..last night I left the vehicle unlocked with the smart key in my pocket for a couple of hours. When I went back to start it, it didn't 'find' the smart key. I held it under steering column etc.. but nothing. The starter switch was fine as radio turned on, but no start.
Anyway, about 30 minutes later, the car & key sync'ed and hey presto it started.
Did I do anything to cure it by pressing lock/unlock on the key fob or was it just due to time the time that the key was back in the car that led to the re-sync'ing? Just in case it happens again, it would be good to know....

If it lost communication you wouldn’t have been able to get anything working. Best guess is that the immobiliser was in play or that the car wouldn’t come out of sleep mode. Locking/unlocking may just have rest everything. Just see what happens. 2019 5.0 SC SVA (Current)
2012 5.0 SC AB. Sadly written off by a campervan
23 plate Focus ST (Current)
1942 VEP Ford GPW Jeep (Willys) (Current)
1943 Whites M16 Halftrack (Current)
16 plate Eurofighter Typhoon, 2 x Eurojet engines with 20,000lbs thrust each. 1380mph

I get this randomly and occasionally. I stay in the car, press lock then unlock on the key fob, then place key under steering column as per manual, then hit ignition. That always has worked for me the last 4 years or so, SDV6 2012my.

Thanks both, good to know Father Jack