Yep, my SurfaceBook 3 has mysteriously done the same today - hasn't been taken anywhere or subjected to any stresses, but it now has a 2-3 cm arc hairline crack on the black bezel between the volume button and the front camera.
I have not yet approached Microsoft about this. This is a GBP£2,500 machine that has been extremely well cared for since I got it last summer. If this was my fault or an isolated thing, then fair enough. But it seems to be fairly common, and I would like some action on it while under warranty.
I'll let you know how I get on.
Steve in Glasgow