This was not under a hypervisor, but it did happen as the same time as I was installing some large pieces of software; I'd just recently reinstalled, and was lo

This was not under a hypervisor, but it did happen as the same time as I was installing some large pieces of software; I'd just recently reinstalled, and was looking forward to playing X-COM 2. The 3GB number is how much space was free after I excised the broken shards of the Windows 10 installer. So, sure, the space probably got taken up after the installer checked if there was enough, but it's not like I ever told it to start running.