I know it appeared to be like an accident, but there were three things that really would stick out to me to at least make Scar look bad, actually, depending on if Zazu knew Scar hit him, four:
1.) Nala would recognize Shenzi, Banzai and Ed as the three hyenas that had tried to kill her and Simba the night before. Scar bringing them into the Pride Lands after that would be a HUGE red flag.
2.) The wildebeests - Some of them at least would have to know they were attacked by hyenas, and some of them might also recognize Shenzi, Banzai, and Ed as the attackers so Scar bringing them in, again, red flag.
3.) No body of Simba. There was definitely a body of Mufasa. There should have been one, even if horribly mangled, of Simba. No body, especially tied with the other things, would be a red flag.
4.) Zazu may have recalled being hit by Scar shortly before Mufasa's death. This would also be a red flag to him.
There is actually a fifth one: Simba returned alive after Scar had told the Pridelands that Simba was dead.
Scar had told the Pride that Simba had died. There apparently was SOMETHING done to get them to believe that Simba had died. So when Simba showed up alive, whatever Scar used to convince them that Simba was dead would be shown to be a fraud. If Simba truly HAD lured Mufasa into the gorge to try and off him and become king, why would he run away afterward? And more to the point, how would Scar know his "little secret" even if Simba HAD murdered Mufasa? Simba was SUPPOSED TO BE DEAD! Scar even stated to the Pride Lands that Simba was not dead and acted (genuinely I might add) surprised that he was alive. If Scar had known that Simba had caused Mufasa's death, that he would keep it secret all of these years would be a huge red flag to the Pride, even if brainwashing and PTSD would get Simba to believe the lie. Why not reveal the knowledge of Simba's guilt if he was so guilty?