At the very minimum, you get an FTSA for not activating the target. So you got scored the most lenient way possible. See 9.9.2 in combo with 9.9.3. Specifically, "Activated scoring targets which do not comply with the above criteria are considered disappearing targets and will not incur failure to shoot at or miss penalties, unless they are not activated (Rule 9.9.3 applies). Activated appearing scoring targets will always incur failure to shoot at and miss penalties if a competitor does not activate the target movement either before or with the last shot fired in a course of fire."
In a funny turn of events, you got scored exactly how you intended. Although, it's -25 points not -5 as you calculated (-10 for miss, -10 for FTSA procedural for the disappearing target, -5 for potential hit).
As for your question, let's say there's a long wall perpendicular to your view. Also, let's say that there's a target behind this wall. If you "shoot at" this target, you should get disqualified on the spot. What you did was so strange that it probably didn't compute in the moment, and they gave you the benefit of the doubt.
Three possible results:
-25 points (-10 miss, -10 FTSA for disappearing, -5 potential hit)
-35 points (-10 miss, -10 FTSA for disappearing, -10 FTSA for steel, -5 potential hit)
DQ