From GameWatcher: “Warframe’s Nightwave has sent Tenno across the playable universe, making them complete various side-activities that never really caught on in the meta, so it was only a matter of time before some of the game’s curious quest-specific features got showcased, too. Silver Grove Specters are precisely that.
Warframe Silver Grove Specters first appeared as part of the Titania questline, where they served as a sort of a mini-boss encounter which kept said Warframe’s parts intact over long aeons. It just so happens that they drop some interesting mods, too.”
Source: N4G PC Warframe SIlver Grove Specters – Where to Find Them and How to Kill Them?