I taught my kids a game called "Stop Your Feet!"
And "Now you can go."
And we used it for fun, on the sidewalk & playground, in totally safe places.
And then I used it in the grocery store, the mall, etc.
The penalty for not "stopping their feet" immediately was that they had to walk WITH me, and I held onto their forearm instead of their hand (which is a negative--it's almost as if they're a package instead of a companion). Or they'd have to go in the cart, or hold onto the side bar of it, etc. Their freedom of movement would be GREATLY curtailed
It worked! I could stop them instantly, anywhere.
I also think that "watch out" doesn't mean anything, especially not to a 2yo brain. But "stop your feet" is clear, understandable, and specific. And it bought me time to explain things to them, and to think of other words, etc.
Other short phrases might work as well--if your child is simply wandering too far, maybe you teach him a game called "On Me!" and you teach him that when you say that, he has to come and slap hands with you. That'll get him over by you, and then you can quietly explain to him what you want from him. Or you can simply use it every time he gets a little bit too far.
And there is "inside feet!"
Pick some "crowd-control" phrases, and teach them when you are not at the grocery store. Make them a game.
I did this w/ my son who liked to hide inside the carels at the department store. And then i couldn't see him, so I'd call, but he'd stay hidden. So I explained that he didn't have to come out, he could still have fun, but I needed to know that he was safe. So I'd call out, "Where's my Billy!" in a sing-song way, and he'd say, "here I am!" and that was all I needed. It worked really, really well. I could allow him some freedom and yet still have "control" of him using these pre-decided, pre-practiced phrases.