@HDMI1
Why would they? They were currently in the middle of a mission, plus they likely assumed he still died, so the shield wasn't that great. It isn't like bullet proofing is a new concept to Japan. Sure it isn't expected with their enemy's general tech level, but there always is a possiblity for an outlier here and there given their limited knowledge of the world.
In addition, there are a few key questions which should be asked:
1, What would they even do with/gain from it?
- It likely isn't anything overly special, just either an alloy or stupidly thick. So making time to look jnto it isn't useful at the moment since only 1 guy had it. Also, a shield does fuck all to a tank round or rocket if need be. And if somehow it can, just snipe the guy, then loot the shield.
2, How/Who would carry it?
- That thing likely weighs a good bit. They already are carring a lot of gear, plus their weapon and ammo. Who wants to carry an additional heavy burden? And the shield is massive, so they likely wont fit it inside the tank.
3, Do they need to/is it worth researching?
- Likely no. If they got back and determined it to just be some kind of super heavy grade steel or something, then it isn't worth the cost to bring it home, nor the risk of retrieving it. Someone would have had to leave the tank to get it and either walk it home or ride on top of the tank, completely exposed for the most part. Just because they have a tech advantage doesn't mean accidents still won't happen.
I'm more surprised that the guy was only knocked unconscious. He took a rocket explosion point blank wearing plate armor which bullets could penetrate. The shock wave alone should have him fatally wounded, or at leased with significant internal damage.