mysterious disk image InstallESD.dmg

There is a mysterious disk image on my mid-2011 iMac with High Sierra called InstallESD.dmg. It does not appear in a Finder search so there is no way to find a filepath but Disk Utility can see it and can mount it. Even when mounted, a Finder search won't find it, although once mounted I can see the icon on the Desktop. It cannot be erased with Disk Utility or TechTool Pro, nor can its contents be deleted by dragging to the trash. If I select it on the Desktop and get info, that shows I have read and write permission, but TechTool Pro says I don't have write permission. I can unlock can it but that does not change any of this. It contains one folder called Packages, which contains 4.75 GB of files called Core.pkg, EmbeddedOSFirmware.pkg, FirmwareUpdate.pkg, OSInstall.mpkg, and SecureBoot.pkg. All have file creation date December 14, 2018 and file modified and last opened date July 19, 2019. I want to get rid of this completely; it serves no apparent purpose, and the fact that I can't get rid of it makes me distrust it.