Especially when you are in devfs you should not assume anything at all! Close in devfs is just a function pointer which is overridden by each of the myriad device drivers that expose files in /dev. Your close() could be the final one which lets the driver perform some cleanup. It might decide to borrow your thread to do it. Maybe some device was about to be ejected/disabled but could not previously because you were holding an FD to it.

The same goes for /proc and /sys which are very similar to /dev in that they represent various entry points into the kernel.