Stalker Disappear To Nowhere

[Biogreeza]

ok, so here it goes, me and my teamates are playing a defense mission( i think it is in phobos), just then stalker appear, then we fought him.......but after i escape(bacause having low HP), so my teamates deal with it, the first time i thought stalker went invisible because i can't see him, but then i just realize, he is gone, he didn't even kneel down and disappear in black smoke, or drop something, STALKER just disappear.....if he were invisible, stalker will hit us, but we all are standing there doing nothing and stalker didn't even hit us. i don't know what the problem is, but i am just disappointed. plsease someone explain this to me....thanks~