How to identify which program is using a partition (and is stopping it from being un-mounted)?

Only one partition cannot be unmounted (on other drive than the one the system is installed on) . It is also the only ext4 partition on that drive, the other two are ntfs.

I am using dual boot, have rebooted to fully shut down Windows, all partitions other than the system-running one are unmounted.

In Dolphin it says some app is using the partition

which one could that be? (Dropbox and such are shut too)

Solution based on Rizwind's comment: in order to see which program is accessing the partition, lsof | grep /path/to/partition.

(In this case appimage programs administered by AppImageLauncherSettings were the culprits. Moving them to another location solved this after reboot.)