The marker is of type empty which is why it does not show on your map. You can change the type (ie the icon) of a marker during a mission with a setmarkertype command. I don't think you can select empty when you place it in the editor, you need to do it in init.sqs or somewhere.
If you group a unit to a marker (or a number of markers) it will make the unit start at either his placed position or one of the marker positions, chosen randomly. This is how I created the multiple start positions in the now famous Un-Impossible Mission. (Well, famous for a beta.
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Can't help you on the dead body code. I never bothered hiding dead bodies after a while playing CWC, I doubt that BIS ever got round to writing the detection scripts.