As a certified Entomologist I can tell you that the behaviour of the Formicidae culus is aggressively expansive, they will not stop because a scout has not returned. They will instead send more ants out to find the cause of this ant's disappearance, be it predators, dangerous landscape, or more importantly, a rivaling ant colony. Colonies nearby compete for resources, and so if they suspect the ant has been killed off they may send a fleet of soldier ants out to the place where it had been sent and if they find a nest they attack it, and if they don't they report back with a message that the area is safe and ready for expansion.
The best thing you can do is go to your local lawncare store or closest thing to it which sells anti-insect, specifically ant, pesticides which you can lay down around your lawn. These usually work by simulating food which the ants will then bring back for their queen and colony, and poison them. As well get some to treat any colonies you find in your yard.
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It's terrifying how organized these creatures are...