Tracking down lost ghost locations in Destiny 2's The Final Shape expansion might not seem like the highest priority when the Witness is set to rewrite all of creation, but doing so will complete the Spirit Guide seasonal triumph, plus get you some coveted red-border destination weapons for the trouble. 

You won't unlock these mini-quests until you finish the main campaign and start the Ergo Sum exotic quest that appears immediately after, plus you only seem to be able to find one lost ghost in the Pale Heart right now. Similar to the Neomuna action figures from Lightfall, these quests look to be time-gated, so you won't be able to do that seasonal challenge straight away. Still, if each gives a red border destination weapon, it's worth the time. Check back in subsequent weeks for more lost ghost locations.

Convalescence: Rootbound lost ghost location

You'll first unlock the ability to track lost ghosts after you complete The Final Shape campaign and the first mission of the Ergo Sum exotic quest, Queens 1. After that mission, you'll transmat in front of Micah-10's Lost Ghost Analytics. Interact with the console to begin the Convalescence questline.

The first quest, Convalescence: Rootbound, sees you tracking a lost ghost to The Forgotten Deep lost sector. You will have been here during the campaign, but if you can't remember how to get there, use the icon on the map indicating the search area—simply activate this and follow the nav markers to find the entrance. 

Fight through the lost sector to the final room and then look for the darkly-glowing ghost in between the massive tree and the platform with the well of bright green light. Interact with the ghost and bring it back to the Arbor of Light—just next to Micah's Conduit—and get your reward of a red-border weapon from Micah.

After this the Convalescence questline veers off into doing other stuff, but if you interact with Lost Ghost Analytics, you'll see that Micah-10 has a different card for finding lost ghosts called Ghost Tracker. It explains that you need to come back later once Micah has gathered more info, which is likely to be after the next reset on Tuesday.

Also, before you go running off into other lost sectors to look for more lost ghosts, I already checked and sadly it looks like we have to wait for the ghost tracker.