Park up next to them and start tucking in to a sandwich or sausage roll.....dog will be slavering all over the seats. Then see the look on the face of its owner when he gets back
Never done this myself, but in my head it would be funny
Dogs do communicate in their own little ways.
Ours has a way of gruffing and gesturing with his head for things, particularly when he wants his hair brushing (are metrosexual dogs a thing?)
"Janet has just nipped in for some milk and ciggies, but I was enjoying Top 10 at 10 on the radio so decided to wait in the car know worrra mean, mate?"