Ok, this thing has happened to me in a few occasions in the past several months.

Something really, very strange.
Someone that isn't on any of my contact/allow/block lists signs in (always the same person). When I click the notification, Messenger says the regular we were unable to start convo.. message. I can see the nickname and DP of that user just as a normal contact has signed in. There is no contact with that name on my list. (and I'm not filtering contacts on anything) When I type the first few letters of the contact to the search bar (the one we can use to find a contact on our list quickly), I can see the contact as it was on my list, with an offline status, just like if it were a regular offline contact. I can see their email by hovering etc. normally. Ok, when I double click, I get the regular "add contact" dialog, with the address bar filled with their address (just as if I clicked add new contact and typed their address waiting to click the next button). I quadruple checked that this contact is not on my allow list, or my block list, or my contact list (I am not a dumb guy obviously I am a programmer so please don't come with dumb questions).

I have my own domain with my own e-mail address from that domain, and I've locked that domain for registering with a Live ID at the first day I registered it. That is also my messenger address, if it makes any sense.

I am Turkish, and this person also has a Turkish name with a normal looking hotmail address for that name. I don't know anyone with that name, never met, never removed (and never even added, or never remember them adding me etc.) such a person.

Who is this person, and how can they (apparantly she) can send a phantom-sign-in to my messenger client, and how can Messenger servers allow this? (I googled that email address etc but nothing came up..)

This is getting quite strange, any ideas?