I lost DD2 a couple of times; she was a runner as a toddler and completely oblivious when she was older. I lost her once on the way home from school. She was a couple of feet in front of me, and I was watching her. I looked away for a couple of seconds when DD1 asked me a question, to look at DD1 and answer her question. When I looked back, DD2 was gone.
She'd ducked into the park, deciding to walk home a different way, and being oblivious, she just assumed we had seen and would follow. By the time she realised we weren't behind her, she was almost home, so she thought she might as well just keep going. Meanwhile, DD1 and I had gone back to the school, into all the shops, checked the play area at the park...
On another occasion, she caused an entire hotel to go into lockdown. She'd come back from the kids club and was having a tantrum that they'd turned off the hot chocolate machine early. Because of the tantrum, we were not the only ones watching her. All of a sudden, she made a run for the reception and then vanished. No one saw which way she went when she ran into reception, and the CCTV only covered the exit. We knew she had not left the hotel, but even with dozens of people scouring the hotel for her, we could not find her. Eventually, the staff made her a hot chocolate, and everyone went around the hotel calling out to her that her hot chocolate was getting cold, and she appeared as quickly as she had vanished. To this day, we still don't know where she was hiding and she cannot remember to tell us.
The school also lost her when she went to London in Y6. She wandered into a shop without thinking of telling anyone. It took them 20 minutes to find her.
If you've never lost a child for a short time, that's probably more luck than anything else.
Putting a tracker in DD2 might have been tempting if it were an option, but I doubt I would have actually gone through with it.