Thanks all for your input,
Thanks especially to Worldeater who's solution worked right away... kind of.
My conclusion:
It seems that if you want to have a Detect Dead Bodies script that checks different combinations of variables, who died and who found the dead body, config based sidechecking works best.
Just like Worldeater indicated it doesn't work so well with checking sides on resistance/independent and civilians.
If you want the script to work even on units that have had their side changed (through mission.sqm or grouped with a leader from another side), you better change the actual script conditions to consider:
if (group finder == myChangedSideGroup ) then blahblah
or if (finder == mySpecialUnit) then blahblah
Think I'll go with the config sidecheck after all, I was wishing to cover everything, but I guess it's not possible.
Cheers,
Laggy