When a space marine reaches level 3, he can equip the "Proximity mine frag".
You can use this mine to kill some xenos or in a better way: if you put the mine just in front of the Genestealers spawn, they will not spawn again. Exaclty like when you put a marine in that position.
Leave a marine in that position is a waste, but it is a good deal for a mine. This also works with "Power field generator".
I don't know if this is a bug, or the specific intention of the developers.
That's all.