How does a possession charge work?

My husband and I were moving out of a house, the landlord called the cops because we were not out on September 15th at 5:00 p.m. My kids were the only ones in the house and the cops pointed guns at my kids and then we, my husband and I, came home and the cops said that they can smell pot. We didn't smoke any pot in our house but had a couple of used pipes and a lot of unused pipes (my husband previously owned a head shop). Well they took only two used ones and all the unopened rolling papers. They left everything else. They didn't tell us they are going to press charges or anything. A month later we get a letter from a lawyer saying he is looking at a possession charge. Can they do that?