The HUD in my S60 has been failing to display – it fails to show when starting the car and remains off.
It has been very hard to diagnose as it doesn't happen very often. However:

it fails only when the stop/start function is operating (ie. stop/start i on and is working);
conversely, it never fails when stop/start is inactive (ie. is on but not operating because battery is low);
perhaps coincidentally, it has only failed after a long drive e.g. 100km;
my memory is not good on this but, it seems to fail if, after stopping the car (e.g. for a coffee after a long drive), I forget to switch off the ignition (i.e. flick the starter knob);
also, it also fails if after stopping, I remember to switch off the ignition but start to use accessories such as the radio and/or windows;
sometimes, the HUD does show but it blinks 2/3 times before steadying – sometimes the HUD display moves from the bottom of the HUD screen up to the normal position;
when the HUD is failing to display, cycling the HUD setting in Sensus has no effect;
if I stop, switch off ignition without using an accessory and park for the night, the HUD returns to normal behaviour the next day.

Yesterday, I tried to replicate the sequence of stopping, switching off ignition, using accessories, restarting car. But, the HUD showed normally. This makes me think there is a time element involved i.e. the car has to be stationary for a time with the ignition left on or an accessory left on.

I reported it to the dealer earlier this year and they found no problem.

Has anyone seen behaviour like this ?