It will come back through your faucets. If it happens when your faucets are closed, then there is a problem with one of your shower/sink valves. If you are using any of the faucets then you are simple allowing the faucet to complete the loop and you will have cold water coming back down the hot water line.
Make sense? You are isolated where the tankless was, but any fixture in your house that is a hot/cold is completing that isolation you thought you had.