red nova dragon cant be destroyed by effects of cards your opponents controls. scrap searcher destroys all scrap monsters on your side of the field which are non scraps. thus, if scrap searcher got summoned to my side of the field and i have red nova dragon. red nova dragon gets destroyed cuz at that moment its on my side of the field.
red nova dragon precisely says, ''This card cannot be destroyed by the effects of your opponent's Spells, Traps, or monsters.''. at that moment your opponent doesnt control the scrap searcher, so you cant say its his card tough, he is the respective owner of the card, but isnt declared while on the field. red nova dragons text seems a bit confusion as it didnt said "control".
example:
- i got RND
- you got Scrap Golem
- eff SG summon Scrap Searcher to my side of the field
- SS applies, RND dies
i hope this clears up the confusion.