Grimm can open private "windows" in space which allow him to see into unprotected areas, assuming he knows exactly where he should be looking. Once he has familiarized himself with an area, he can open a portal there which he may physically enter to transport himself. His windows are invisible to anyone but himself, or those who have entered his mind. The portals are visible on his side, but invisible on the other end, this allows him to enter locations stealthily. 

This does not tell me anything about the process. This tells me what it can do. That is why I am trying to figure out how its happening. 

But im done talking about it, as I don't want you to think im attacking you or being an ass.