Nintendo Life posted a photo of a slight hairline crack on the back of their Nintendo Switch console on Twitter and got a fairly big response from others with similar flaws detected. The cracks seem to appear in a similar place on the right side of the back of the console, and many point to heat expansion as a likely cause.
We quickly polled our staff, but found no cracks in our units, and the issue may be a fairly small one, or perhaps related to how you use the console (one of us had noticed a slight warp of the shape on the back of the unit), whether you tend to use the dock a lot (where the unit is known to become quite hot).
Have you noticed any similar cracks on the back of your console? If so, you may want to check and see if your warranty is still in play.