I have put several objects on the ground, but only the dagger was "lost", all others were retrieved normaly.

I have not report a bug because I can not provide enough details, I have posted this here because if somebody else had a similar case, with self made things.

I have noticing something different also, that if i use magic spell(weaknes, frost), that "eats" the monsters HP, I can not loot the corpse, I must kill(last remaining HP) the monster with weapon or some direct spell(summon arrow,...)

No there was nobody around i have checked, unless there is some spell that can make someone invisible.