If you place a fx explosions to go off at a certain time, like a reach marker or after scanning something the explosion does occur as planned, however i have noticed that tiny mini explosions often occur before you have completed the goal to set them off. I know objects are always on the map and its just a case of when they are set to appear but explosions have a tendency to appear too early.
its not the full explosion but it can give the game away that explosions are coming.
Is this in testing or once the mission is published? I noticed last night that the behavior is different in the testing engine versus once the mission is live. My explosions etc would "go off immediately slightly then hide" when I loaded into my mission. Once I published the mission it worked properly though.
This has happened to me, as well, but I thought it was because the space explosion animation is the same one we have when fighting ships. When you've destroyed a ship, there is the small explosion, followed by the larger one.