Should newbies be ok with this practice? is it fairly common? Why a lot of providers do this?

It's fairly common I think.  Many girls are dressed very scantily or not at all when you arrive and they don't want to just leave the door ajar.  By hiding behind the door, they can welcome you dressed any way they want to, and be ready to give you a hello hug and kiss.

Don't worry they aren't doing it to jump out and rob you, nor are many of them doing it to bait you into the room (although that does happen on rare occasions) it's just that they don't want to be exposing themselves to someone besides you that could be walking down the hallway as they answer the door.

When I go see a provider, I always make sure the hallway is empty when I knock on the door. It never crossed my mind that girls would be hiding from a possible passer by. But it makes sense now.

I'm fine with the 100% mystery thing but I think I know what the OP might mean.  Even if someone is nekkid, opening the door to sign for a FedEx or to let the dog in, you usually hold the door tightly and stick your head or face around the edge to make eye contact or see who is there.  It sort of seems natural to greet who is at the door face to face or half-face to face, kind of like in the pic, below.  

...you talk like you're a provider (see the link) now you're talking like you're a monger. Which is it ?

I don't see what's wrong with that or why you want to know if im a provider or client.

-- Modified on 8/29/2015 10:05:09 PM

...for using an alias. You'll notice I didn't say anything about your using an alias. (mmmm feeling guilty are you?) Whether you are a provider or a client doesn't matter either. Posting as one and then the other is a horse of a different color however. Deception raises many questions regarding your purpose for being here. Just what is it you are trying to accomplish? What are you hiding and why?

Another possibility is that Provider-Inckognito and Client-Inckognito are sharing one account.  That is not only deceptive but I think it is against TER rules and is a bannable offense.

I also thought perhaps this person uses his/her non-alias account for posting/reviewing as a client and intended the alias account to be for posting (POSING??) as a provider....

The lady may want to add to the mystery a bit or make sure no one in the hallway sees you walking in. For whatever reason, its normal. I do it to make sure no one in the hallway sees me for both of our privacy.

inck ognito?

Girls also call call girls you know that right? Expand ur mind. Not only men call hookers. And they always hide behind the door, heck even some guys do this. And when I go on outcalls I always check the hallway is empty.