Just got an 855 frame, and I really hope this isn't a crack  They are where the "seat tube" meets the boxy part that the swingarm bolts onto, they are about as thin as a hair, thinner than a fingernail. Can kind of see the back of this weld through the big triangular cutout in the back of that boxy piece and there does not appear to be anything showing through the back, taking the bike out and jumping/bunny hopping/pedaling hard did not make them any bigger or wider. No other damage of any kind and it does not continue even a quarter of the way down the long weld there at the "seat tube". Is it anything I need to get welded immediately? Or just keep an eye on it?

Update, oh crap, I can actually see them from the back    But light does not shine through them, and they aren't really even on a weld which is confusing? They don't seem to be getting any bigger or anything, should I just keep an eye on them and see what happens? Sorry for making all the posts, really do not want a broken frame in my future!

if it is a crack i would imagine it will creak like hell

It is a crack and it goes all the way through, but it does not creak at all (even when bouncing around on the bike) and it doesn't seem to be getting any longer/wider. It really has me confused...

Aluminum doesn't have a repetitive-stress threshold -- if you tap on it with your finger enough times, it will eventually crack and fail. All aluminum bike frames have a finite lifespan because of this.

The fact that light doesn't shine through just means the frame hasn't stretched where the crack is, or the crack goes diagonally through the metal so light can't shine through in a straight line. But it doesn't matter if it doesn't creak or light doesn't shine through, all that matters is the metal is cracked. You either need to find someone who can weld aluminum, or you need to buy a new frame. Those are your only two options that don't involve severe injury.