Those 2 body bags were people that fell out of the plane crash, it was in season 2. It was filmed intentionally vague so you think it's Walt or his family though.
Something cool about season 2, if you read the names of the episodes that had that black-and-white foreshadowing it tells you exactly how the season is going to end:
1.
"737" (the type of airplane it was, also the amount $737,000 that Walt needs before he dies)
2. "Grilled"
3. "Bit by a Dead Bee"
4.
"Down"
5. "Breakage"
6. "Peekaboo"
7. "Negro y Azul"
8. "Better Call Saul"
9. "4 Days Out"
10.
"Over"
11. "Mandala"
12. "Phoenix"
13.
"ABQ"
Gus didn't necessarily know his car had a bomb, but he knows he didn't poison the kid. So when Jesse won't cook, calls him to the hospital, then says "yeah, this kid was poisoned" he immediately thinks Walt had something to do with it. Who else would interrupt the cook?
That makes him nervous because his car was left unattended while he was talking to Jesse.
One last thing. The plant they show at the end of season 4, proving Walt poisoned the kid....did you notice it in episode 12?
The exact scene from episode 12:
http://www.youtube.com/watch?feature=player_embedded&v=_rzjS4hzR8E
That's where he gets the idea to poison the kid.