How to keep tour guests from wandering off during stops

Guests leaving the group for "just a quick stop" can turn a smooth tour day into a tracking job. It happens at markets, photo stops, food halls, restrooms, and anywhere the group has ten minutes of loose time. One person wants coffee, another wants a souvenir, and suddenly the guide is counting heads while the driver is watching the clock. I have learned that vague instructions are the enemy. "Me…