I think they can actually see the chests through walls
Some outlanders can. After you use your pick five times, you are in the zone, which means you do more damage with your pick, and you can see chests and anything lootable that is at least a green, through walls. You have to be fairly close to it, but with upgrades you can see things even further. Again, it's not every outlander, but it appears to be a pretty common ability, and will frequently lead you to attics and basements that have no usually access besides just chewing through the ceiling of floor. I found a pair of two vaults (one glowing purple, the other blue) cleverly hidden behind a pair of bookcases in a house.
Unrelated PSA: DON'T SLEEP ON UPGRADING YOUR SURVIVORS.
I got a llama from the collections page filled with survivor xp chips, so I decided to boost up the ones I had slotted. I went from 10 power to 14 in about four minutes. Huge upgrade. Do it.