I have seen this thing in like 2-3 locations late game. I think I picked it up once. But I have no clue what it is? Its always next to ammo, health, or armor. Usually just walk over it and it stays there, but sure I picked it up once.

Ammo for siphon grenades.

Those need ammo? Dont those use cool down like the others? Either way thanks for the response.

@john1912: They do recharge, but picking these up will recharge them instantly. Grenades and the holograms also have weird looking ammo packs lying around. I mistook the hologram ammo for a bathroom sign when I first saw it.

Cool, was really bugging me. Cuz it looks kinda cool and important! Tried to look it up, but didnt see anything. Not on the ammo list here http://doomwiki.org/wiki/Supplies, and checked the siphon grenades section of the same wiki and doesnt say anything about that either. They should update that shit! Thanks again!

Got to level 9 before I realised they recharge! Mixed signals...

Yea, not super clear! Esp that there are ammo pick ups for them!

Actually I am still weired out. The pic I took is at the VERY start of the stage. There are no enemies to fight. You have to jump off a high cliff to enter the next section, other then to take pot shots at two imps and a hell knight. Which kinda makes the ammo pick up for grenades pointless. I would think your cool downs would be reset, even if they were not, again pretty pointless.

@john1912: I'm pretty sure when you first pick up a grenade it pops up a tool tip saying that equipment items operate on a recharge. I never found the pickups all that useful since the recharge time isn't THAT bad, and once you have a healthy amount of weapon options, there's not much need for being reliant on grenades.