So basically, I think Maedhros would not have expected Fingon to come, as he saw the ships burn and no one in their right mind would cross a frozen wasteland (tbh I agree).
And there's harp, and songs.
All fits too well with the only person that could come to rescue - renowned Noldor musician, his younger brother.
Maedhros is 100% clear that for ANYONE to come for him it was a bad idea; he understood; he would say no himself.
But deep in his mind there's still faint hope that his family might eventually find a way to reach him; afterall it had been 30 years...
So we can't really blame him for making the mistake.
And Maedhros kept this to himself. Of course he would.
But maybe it was when he's half-conscious, or when he's drunk (how else could he pass all those long nights after the Tears Unnumbered?)
He might blurt out, and Maglor would hear this.
"I thought it was you."
Feel free to think this piece as Russingon or Mae/Mag or everyone are just frends&family. I ship both so wanted to leave this for readers to decide.