i have seen quite a few here mention that they leave the engine running while doing a 15 minute walk around check,that seem`s alarming,as some toe rag can drive off with your motor,while you are the back,checking the trailer lights,and all the other legal checks.
When the old bill,find it,the insurance will know that the keys were left in it,i always have a spare set of keys,to lock the cab,there is a chance,somebody will rob your cab contents,and think of the money,that is invested in the tramping kit.
I always thought it was an offence to get out of a vehicle whilst leaving the engine running
if it takes someone 15mins to walk around their truck its either huge or they need to see a doctor about their pace 
In a dodgy area,it is a risk some thief will get in the cab,say like Marseille,Lyon,or Paris and it`s suberbs.
acd1202:
I always thought it was an offence to get out of a vehicle whilst leaving the engine running
It is 
Insurance will not pay out.Ask the gaffer for some spare keys,they are handy when you get jailed at a RDC locked out of your legal rest place/home/work place,that is your cab,when they ask for the keys.