What's it doing behind my back?

Windows 10 Pro 1709 (16299.248)

Even when I am doing nothing on my system, my internet activity light on my
router port keeps blinking merrily away. As a test, I put my system to
sleep last night. After the screens went dark and the keyboard LEDs turned
off, my disk activity light and the aforementioned activity light kept on
for over two minutes. All my disks are set for immediate writes, so there
should be little if any disk buffering to finish writing. So what is
Windows doing for those two minutes that results in continuous disk
activity? And yes, the internet activity light finally went out at the same
time the disk activity one finally went dark.