For example, flagging animal corpses so you can find that deer you shot in the dense forest easier. There could be a message saying how far away it is like in the look menu. Could also set it to alert you to any specific item, so you can track down things easier in cluttered stores.
First thing that came to mind when reading this was an alert when you see say radioactive waste (called (watery) plutonium slurry ingame and this can actually contaminate you, but only if you pick it up as far as I know), but this is a very niche usage.
I was thinking more along the lines of looking for specific components in a cluttered store where shelves block LoS. Instead of walking a few steps and pressing V a bunch of times you can just walk and it will alert you if you can see something you have flagged.
you can turn on auto pickup for items you want on a list
unless you’re willing to do some coding to reverse engineer the “warning” system its the closest you might get
What a coincidence, i was thinking about this “issue” few days ago. And yes i do agree to implement a warning system for itens that you have flagged, similar to how the auto pick-up works for NPC, mixed with the monster warning system.