What are these pickups?
I came across these a few times and I wasn't able to pick them up, even though none of my weapons had full ammunition, and even my health wasn't full.
Are they ammunition for a weapon I don't have yet?
It's a Hologram Grenade. For proof, check out this longplay at 3:03:40. That explains why you couldn't pick it up, from your screenshot you already have a hologram prepped in your inventory.