I have two quests outstanding called:

* Find the truth about Tingalf
* Find the suppliers of Tingalf

I have dispatched Pedro, found the letter, reported all this to Marcus etc. None of Trevor, Marcus or the Rivertown watch captain (Roberts is it?) have anything more to say to me on the matter. Tingalf himself is no longer in his corner near the market and I assume he's been arrested (I think someone mentioned as much at some point) so I don't understand why I have these two quests outstanding or what I'm supposed to do about them.

Help!

I just noticed I also have a quest called "Find clues a bout the wares" outstanding. I am very confused about this.

This series of quests can be a little sensitive to doing things too much out of the expected order. As long as you got the experience rewards, and Tingalf has been arrested, the remaining quest entries are just a cosmetic problem (and will be cleared late in the game).