Windows sometimes show 100% disk usage when no apps are running

The reason Resource Monitor isn't pinning it on anything can be the clue here. When the disk sits at 100% "active time" but the actual read/write numbers (Total B/sec) are small, it usually means the drive itself is the bottleneck, not some hidden app. 100% active time just means the disk is never idle, it doesn't require heavy throughput, so a slow or struggling drive can show 100% while barely moving data. That's exactly the case where nothing obvious shows up as the culprit.

I'd check the drive's health before chasing processes. Open PowerShell as admin and run:

Get-PhysicalDisk | Select-Object FriendlyName, MediaType, HealthStatus, OperationalStatus

HealthStatus usually says "Healthy", but don't fully trust that on its own, it only flips to Warning quite late when a drive is already failing. The read/write error counts are the better tell:

Get-PhysicalDisk | Get-StorageReliabilityCounter | Select-Object Wear, ReadErrorsTotal, WriteErrorsTotal

High or climbing error counts mean the drive is retrying a lot, which is exactly what spikes active time to 100% with almost no throughput.