Encrypted disk image stored on Volumes disappeared from Mac

Basic question, did you check the Trash? The disc image file would not disappear on its own, although the mounted disk would be unmounted if you ejected it or reboot the Mac. If you inadvertently deleted the file and have emptied the trash, if your Mac is new enough to have an SSD then the file is gone for good unless you have a time machine or other back up from which you can recover it.

A Mounted disk image is stored in a .dmg file somewhere.

If it is no longer mounted, where is its .dmg file?