windows - What is using 20GB of memory & why doesn't it show up in task manager?

Overnight, some unknown memory leak clogged up 20GB/24GB on my system. I've closed everything yet the leak persists. Running latest Windows 10.

How can I track down what is causing it?

• All software up-to-date


• No other users ever use this system


• Nothing stands out in task manager


• Nothing using too much CPU