Yes. The weapons inventory says I have no darts. I've also tried changing from the lady to assassin to lady again and it just doesn't come back. My weapons rack doesn't seem to carry it either so I can't re-equip it and, I can't buy it from the venders.
I can't remember which mission gave it to me in the first place. I was thinking of maybe just redoing that mission to see if it will be returned that way.
Otherwise, I'm at a loss.
I don't know what happened with your game. But the mission you get the parasol in is Sequence 3; Prelude to Rebellion (The first Sequence 3 mission). You don't actually use the parasol until the next mission. Hopefully replaying the mission will get you the weapon back.