Each K9 and handler team has a set signal for when the object that is being looked for is found. Different objects have a different signal. In most cases, they’re broken up into two types: an active signal and a passive one.
For contraband, drugs, and people, they’ll paw at the spot they smell it. If it’s something combustable or dangerous, then a passive alert is used, where the dog just lays down, so they don’t disturb the object.