Probably looking up the dominoes is enough to make it "busy".
I had always assumed it was location tracking data, same as they (mostly?) use for realtime traffic data.
But, now that you mention it, Dominoes is a delivery business. So you'd have to estimate their current volume some other way - a spike in demand won't correlate with an increase of people physically visiting the storefront.
We'll probably never get it, though? Seems like one of those algorithmic things Google holds close to its chest.
I would think, though, that if it's going to be meaningful whatsoever for a delivery business... it couldn't rely solely on "phones in this area per hour." But I'm just a random yahoo speculating.