Talk:The Grustrag Three/@comment-70.138.35.151-20140316003100/@comment-24557530-20140316112348

Once you got the mark, it will not dissapear until they visited you.

If you have a mark there is a certian chance (3.5% for the stalker, i do not know if it is the same here) that the spawn if you play a non-boss mission on a grineer maptile, with grineer-units actually spawning (so Invasion is also fine, as long as you play on a grineer maptile).