In this context it means that you don't even notice it. For example a friend of mine got a cochlear implant. He had been deaf before getting it. He would say "Wow guys did you hear that?" And everyone would wonder what he was hearing. Turns out it was birds but since everyone else had heard them time and time again they didn't even register to our ears until someone pointed them out.
The word can mean more than this however but in this instance it means that they didn't notice.