Passive vs. Aggressive Alerting
When a dog finds what he's sniffing for, he lets his handler know it's there by giving the alert signal. Drug dogs use an aggressive alert -- they dig and paw at the spot where they smell the drugs, trying to get at the toy they think is waiting there.

Photo courtesy U.S. Customs/James R Tourtellotte U.S.D.A. Beagle

However, there are some specialties where an aggressive alert would be bad news. If a dog searching for a bomb digs and scratches at it when he finds it, the results could be disastrous. In these cases, a passive alert is used. A good example of passive-alert dogs is the Beagle Brigade used by the U.S. Department of Agriculture to sniff out produce that isn't allowed to enter the country. Because no one wants a dog digging at their belongings, the USDA beagles have been trained to simply sit down when they smell fruits or vegetables.