I'd suspect that the grenade is actually made of a degradable substance of some sort, or designed so that the molecular cohesion breaks down after a couple of hours or days. If for no other reason than you wouldn't want to risk your own troops using them on the enemy, taking that position, and then a couple of them "tripping" over the loose line on the ground and cutting their own feet off.
Man, I'm going to have FUN with these if any player of mine ever decides to use them They're a little bit like Shadowruns version of a Deck of Many things. Very cool when they work, and they suck balls when they do not.
Bull