No condescension taken! However, I am pretty sure that "se le dejó" doesn't work in this context. As you said, "le" is accepted for direct objects. So you *could* say "se le dejó en las escaleras" instead of "se lo dejó en las escaleras." But the meaning of "dejarse" is closer to accidentally leaving something behind than simply leaving someone or something. It would make sense to say "se lo dejó en las escaleras" if you were talking about someone forgetting their phone on the stairs, but I think "se le dejó en las escaleras" would only work if you were talking about someone leaving their son at the stairs by accident or something, and even that would be a pretty colloquial construction.

And I hope that doesn't come across as condescending! Maybe we can get a native speaker to weigh in on this.