If the shelter has a policy of keeping animals with collars or tags longer than those without, don't assume that your pet still has its collar on. Ask shelter personnel and volunteer staff for a list of any other rescue organizations. You should also check surrounding animal shelters in the area your pet was lost . I once consulted with a woman who wasn't aware of a private shelter in her area. She went to the shelter to check and there was her black lab in the first cage, up for adoption.
Many pet owners make the critical mistake of only checking their local pound.
If your house is located near the border of two different counties or towns, your pet may have been taken to either shelter. Look for other rescue organizations to contact. Once inside the animal shelter, inspect the areas where injured or quarantined strays are kept . If your pet is sick, injured, or has bitten someone, it would be housed separately instead of in the general holding cages. Some shelters have this room out of sight.