I think it just depends on the dog and handler, they can indicate a "find" in whatever way they are trained too. These are called passive search dogs, trained to work as the "target" walks past. There are also active search dogs, that would be sent into a cell, say, to actively search the cell. They can be trained for all sorts of things drugs, explosives, fire starting chemicals (to detect arson), in the prisons they are trained on mobile phones, SIM cards etc. They are clever as hell.