We do that too edam - ever since a traumatic experience in York Railway Museum when we abruptly realised (dd2 was a few weeks old & in a sling) that we'd been terribly complacent with just ds & dd1, & that 3 is a Whole New Ball Game.
It's a horrible experience when you lose one. Dd1, aged 5, went AWOL in Leeds Ikea last year as dh was taking her & dd2 to the loo & I was arguing about something inconsequential with ds.
Ds & I proceeded to the food bit, still arguing, & assuming dh had both dds.
Nope. Dd1 had come out of the toilet to find me, whilst dh was still tidying up dd2, failed to see me - & I wasn't looking out for her, too absorbed in talking to ds - so decided she'd better run back to where she'd last seen me.
It took about 5 minutes for dh & I to realise she was missing, as I was getting food with ds with no idea that dh thought dd1 was with us, & dh & dd2 went to get a table, assuming she'd come out of the loo & found us.
By that time dd1 was wandering in a state of panic around the sofa section. The thing is with dd1, her version of panic is pretty contained, & of course there are kids everywhere, so she could easily have been ignored. Luckily, a kind couple spotted the way she was brimming with tears
& took her back to staff at the entrance just as dh & I had realised she was with neither of us & were searching the restaurant & toilets.
Suddenly there was an announcement over the tannoy: 'Can Mrs Raven come to the front entrance to meet her dd, please?'
The staff explained that they try to avoid broadcasting 'lost child'...
The couple who'd found her (to whom I am everlastingly grateful!) had had no idea what to do, though - they suspected, correctly, that by taking her to the entrance of the huge store they were taking her further away from us, but didn't know what else to do.
We've since drilled into all 3dc that the thing to do is approach someone in staff uniform & say 'I'm lost!', & make damn sure we both know which dc are with each of us. Scary stuff.