I left him looking at the fish at the fish counter and told him to stay there as I was just going to get some packaged salmon which was a few paces away. I turned back and he was gone. Not in any of the aisles nearby. Cue mild panic arising.....I shouted his name really loudly a few times and expected him to come running, or shout me, but he didnt.
Was getting more panicked by this time remembering that poor little girl in Germany years back who disappeared in a supermarket and has never been seen since. I was also thinking "God, now if there's some wierdo perv witnessing this, they now know my son's name and if they find him first and use his name he'll assume they're safe to go off with."
I told a couple of staff members who went to alert security at the door in case he'd wandered out. By this time people were starting to stare at me running up and down the aisles looking worried sick and shouting his name, and a lovely lady shopper came and said "have you checked the toy aisle - that's where mine have always been if they wander off? I'll go and see if he's there."
Eventually after what seemed like ages but was probably only about 5 mins after he'd gone missing, one of the members of staff motioned to me to come down to the tills, and there he was with the lady shopper who'd suggested I check the toy aisle.
Such relief!!!! Apparently he'd gone looking at the Nintendo DS's. 
After thanking the ladies who'd helped find him, and letting my legs return to their normal unjelly like state (!), I told him that he shouldn't wander off like that as it was very dangerous.
"But I couldnt see you so I went to look at the DSs."
He obviously had no plan of what he was going to do after that if he still couldn't see me. And absolutely no awareness of the dangers of wandering off on his own.
DS1 was never like that - he was born sensible and stuck to my side like glue in shops etc at that age. He just NEVER would have wandered off to a far part of a shop to look at something just cos he felt like it. DS2 is too big to sit in the trolley now, but is naturally bounding about with energy and will never walk if he can run - short of putting him on a lead I don't know what I can do. He just doesn't seem to see the DANGER of getting separated from me, he thinks he's invincible I think! After he went missing today I tried telling him that a nasty man could have seen he was on his own and taken him away and smacked him hard every day and he would never see me and the rest of the family again, and he just said "But there are no nasty men in Tesco, mummy!"
How on earth do you get a child who is the opposite of sensible to be aware of "stranger danger"?!??
Another example of "opposite to sensible": the other day we were in the park and I told him to mind the nettles which we were walking past because they would sting if he touched them. "They won't" he said, and put his hand slowly to them all the while staring me in the eye defiantly. He DID touch one even though I was telling him not to, and either it wasn't a nettle (looked v like one) or it was part of the leaf which didn't have any stinging hairs on. So he just said "See - didn't sting me!"
He just has no sense of self-preservation!