"You can survive for a couple of days without water, a lot longer without a mobile phone".
That is true but its a bit like the emergency bag - unless you carry it around with you and that's not practical (I have seen Eric's list) you have to decide what is the most important things you want with you where ever you are.
My list is as follows:
- Water - although how much is enough especially in this heat
- Phone (plus charger) I live in a remote area that isn't densely populated and it probably wouldn't be a high priority area in the event of something so serious it required intervention from the emergency services and I like to think if I could make contact
at some point at least some family and friends who would come over to dig me out

- Passport/Kimlik/Permit so I can leave the country for a while if necessary
- Keys - one of my house doors needs a key to open it so I have to carry it if I am on that floor.
- Credit cards and cash to purchase water if I am not actually buried somewhere and there is plenty of bottled water to loot in the shops if necessary

- Whistle - yet to purchase
I know it is not totally adequate but it is what I can realistically carry with me everywhere I go. As for the rest I am sure survival instincts will kick in and we'll all do what ever is necessary. I hope we never have to find out
