This message is shown when Dell (Insyde) firmware is trying to PXE-boot from the network, in which the first step is of course to verify the Ethernet connection – the 'media'. This can take up to several seconds when there's no cable, and Dell does it once for the IPv4-capable loader, and again for the IPv6-capable one.
So its presence means the UEFI boot order got reshuffled and somehow the built-in “Onboard LAN IPv4” and “Onboard LAN IPv6” entries have highest priority.
(Usually the boot order on Dell UEFI systems looks like this: custom OS entry; built-in “PXE” entries; built-in fallback “HDD” entries; built-in “BIOS mode” (CSM) entries.)
What it also very likely means is that the custom OS-prepared boot entry has disappeared from the NVRAM, and the computer only boots thanks to the fallback
\EFI\BOOT\BOOTX64.EFI loader that happens to be present.
Fortunately this is easy to repair. On my Arch Linux system (which uses systemd-boot), the convenient way is
bootctl install. Windows 10 systems can be repaired by running
bcdboot c:\windows. The rest – by very carefully using efibootmgr or similar tools.
(Alternatively, of course, the fallback “HDD1-1” boot entry could be moved to the front, before PXE, but that's just lazy.)