Is it illegal to take things from a dumpster from a store such as Kmart or Walmart?

I live in Pennsylvania and a few stores near my house always throw out display models and stuff like that but there isn't any signs or locks on the dumpster saying you can't take it. One time a person at store said they were going to have me arrested but I wanted to know if it would be illegal to look in the other dumpsters? Also could you give my your source? Thanks