Finding the final NES006 sample for Failsafe in Destiny 2 is a little tricky, especially as you don't get a quest to acquire it by simply talking to her. Instead, you'll have to grab a mysterious item that's appeared in the Research Bays menu called the Proximity Sensor Alert. This will help you acquire the sample you need and the quest with it.
Completing this specimen quest also unlocks a way to summon secret bosses in Battlegrounds: Echoes activities, letting you claim more loot at the end, and there's even a possibility to get a rare Assimilated Beacon that allows you to summon three powerful world bosses on Nessus with their own unique rewards.
All that said, here's how to use the Proximity Sensor to find NES006, complete the quest, and how that whole secret boss summoning thing works.
To start the NES006 quest, you'll first need to head to Failsafe in the HELM, enter the Research Bays menu, and pick up the Proximity Sensor Alert item on the right. This won't start a quest, but will let you track the NES006 sample you need to collect to start it.
You can find the sample in any of the three Battlegrounds: Echoes activities. When you get close, you'll see the Proximity Sensor bar on the left side of the screen start to build and you'll hear a beeping noise, indicating that you are close.
Here's where it's located in each activity depending on which comes up when you activate the playlist in the HELM:
As mentioned, the beeping noise and Proximity Sensor bar will make it easy to find once you're close.
Once you have the sample in-hand, head back to Failsafe to get the Specimen ID: NES006 quest. Here are the steps you have to complete:
It's a pretty simple one overall. I'd suggest bringing Khvostov out for the first step, using an Incandescent or Voltshot weapon for the second, and then slotting a Harmonic Siphon mod on your helmet plus Heavy Handed and Firepower on your arms for extra orb generation for the final one.
You'll also want an activity with high enemy density. You could head to Shuro Chi in the Last Wish raid, but if you can't be bothered putting the checkpoint code into the wish wall, I'd recommend doing the Onslaught activity in the Vanguard destination.
The final step is definitely the grindiest, but you'll get bonus progress for orbs created in gambit or crucible, so that'll speed things up. If you want to try hard, you could even do a private crucible match with friends and generate orbs from each other.
Once you complete all steps you can return to Failsafe in the HELM and claim the Assimilated Torch Hammer, which you can use in Battlegrounds: Echoes activities to summon special bosses called Assimilated Demolishers. These will appear when you get close to the vex gates located in each of the activities.
Here's where each gate is located:
If you defeat this boss and then also complete the activity, you'll get extra rewards, but as mentioned above, you also have a chance to get an Assimilated Beacon. If you head to Watcher's Grave on Nessus, you can use this item at a special vex conflux near the giant Cabal land tank. This summons three Assimilated Demolisher bosses.
These are pretty strong and you only have five minutes to defeat them, but once you do it's possible to get one of the rare 400g purple samples that you need to complete the seasonal triumphs.