Fish cant get ich from an ich free environment. It has to be introduced from an infected fish, water etc. Without host fish the ich lifecycle will end at the free swimming theront stage. It cant spontaneously appear.
However healthy fish can usually live with ich just fine. So you could go years with no signs of ich as it repeatedly goes through its lifecycle, a fish suffers from a weakened immune system for some other cause, ich takes advantage and it looks like it suddenly appears.
I would say, unless you have a rigourous quarantine, you likely have ich in your tank. I dont really see how fish stores can avoid it given how they operate, even the best ones. Maybe a bit of a generalisation, but that's why you quarantine.