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By default any dead bodies and wrecked vehicles do not dissapear. So you are either using a script which does it for you or if you are playing a mission to tape the movie from the mission itself uses this script.
You will have to edit the mission, or the script to remove that behaviour.
** thread does not belong in this section, moved to editing/scripting.
Oh btw, really there is no need for you to learn everything in Armed Assault first before moving to Arma 2.
In Arma 2 certain things changed, were improved and made easier.
If I may be so free and you have the possibility I advice you to buy Arma 2 + Arma 2: Operation Arrowhead and have all the good fun