My son is 18 months old. I am not worried as such. I don't envision disaster all the time. In fact, I never contemplate this scenario, as I never do it. I simply prefer to cover my bases before the worst happens, rather than pull my hair afterwards, when there would be nothing I can do.
I apologise for being judgmental, apparently it is a very sensitive subject.
Re petrol stations etc - you never see the car 100% of the time - when you enter, when you queue, when some obstacle hides your car (other car or people standing in front of it), when paying (do you look at the child all the time, even when paying, when talking to the cashier, when entering your pin, when putting the change in your purse? Nope...) And even if you are looking - guess what - you may witness someone snatching your kid and by the time you run out - they are gone! Total nightmare. I don't want to take part in it, thank you very much.
Like others have said - there are other dangers - fire, another car crashing into yours, theft etc etc. I may sound OTT, but am not. I simply feel uncomfortable with the idea of leaving my child alone.