There are about twenty of us in the minyan where I daven and pretty much everyone knows one another. We have no levi and so the same person, a kohen, usually receives both kohen and levi. It annoys me when the gabbai asks, "Is there a levi here? Anyone here a levi?", since we know everyone there and conveys, to me, a cold and impersonal feeling. One the gabbaim likes the practice in case someone visits and we don't notice them. But we're a small place and I can't imagine we wouldn't see someone new, say hello, and ask if he's a kohen/levi. What does your minyan do about this?

I think doing this is called "pro forma." You have to say it even if you already know the answer. It isn't personal.

I think that if the gabbai is on top of things and recognizes all of the daveners, he doesn't have to ask if there are any leviim present. I've been in shuls where they actually ask the kohanim to step out of the room so they can give the aliyahs to the yisroelim present(in the case of 3 yarzheits and they're all yisroelim)

My recollection is that doing this is halachically problematic, but I don't remember where it is discussed.

My understanding as well.

Plus, you never know if someone MIGHT have snuck in unnoticed.

The stealth davener.

or someone new and shy who came late is davening quietly in the corner without anyone having noticed him

Depends whom one asks. There are two teshuvot in the Iggerot Moshe where he allows replacing the kohanim for the sake of a revenue stream or yahrtzeits, that sort of thing: OC 2:34 and OC 3:20. He even cites the Maharam Shik as allowing the kohein to be forcibly removed by the gubment if he doesn't leave on his own. But this is only for Mondays and Thursdays - the Igerot Moshe interprets the gemara that having only a kohein first is me'ikkar ha-din only on shabbat and yom tov.
On the other hand, there are those who question this "minhag hedyotot". They probably don't think halachah changes.

What is the minhag / halacha concerning giving a Levite the first Aliyah in the absence of a Kohen? I have heard that it is a "Siman of Long Life", but not much more than that.

You could very well be right. That's certainly the implication of Orach Chaim 135:6. The Rama says a Levi can be called first, not that he has to. In fact, there are poskim [the nos'ei keilim on the Tur] who say a levi davka should not get the first aliyah. There are two reasons: The Ba'ch says so people won't err and assume he's a kohein. And there's a kabbalistic reason that kohen - levi - yisrael is ke-negged chessed gevurah and tiferet, so levi is gevurah, and it's not appropriate to start with gevurah if it hasn't been balanced by chessed.

What does the Orach Chaim say?




I can't honestly see how this would be a problem as firstly: A Yisroel would be called up for Sheni and not the Levi again; in comparison to the Kohein who would be called up a second time in the absence of a Levi. And secondly: Anyone who is paying reasonable attention to what is going on would surely know that the Levi is being called up in the absence of a Kohein. If not then who cares what he may or may not assume!

The Kabbalistic reason of the Tur (???) "kohen - levi - yisrael is ke-negged chessed gevurah and tiferet" is a nice concept, but is there any foundation of following this in Halacha / Torah L'Moshe B' Sinai?

The Shulchan Aruch in Orach Chaim doesn't entertain the possibility of calling a levi instead of a kohein, just a yisrael. Anyway, take a look at the Beit Yoseph on the Tur - he goes through the basis of the discussion and whether it's even permitted to call a Levi first if there is not kohen. It's not so simple, and revolves around how to spin Rashi.




First of all, it's not the Tur. It's a later opinion, in addition to the opinion in the classic nos'ei keilim to the Tur. There's no way the Tur would say something like that.
Secondly, nobody suggested this is halachah and certainly not torah le-mosheh me-sinai.