So, normally Maven is easy to find in Riften, bump into her all the time.

But, I just completed "Loud and Clear" and was told to go find her and talk to her, which is the start of "Dampened Spirits."

Suddenly, she's nowhere. I checked the inn morning, noon, and night, check her house, all around the town, etc. Can't find her anywhere. Tried for several game-days now and she's just gone.

Same thing happened to Lushton, and so I can't seem to finish the quest about the horse.

And in Solitude I noticed the same thing with the headsman (forget his name) who I was doing a side quest for.

Is this a bug, disappearing NPC's? I did a little digging and haven't seen anything. OR can someone tell me where to find her?

I found her on the second floor at the Bee and Barb in Riften. Not sure if you checked there but that's where I saw her and started the next Thieves Guild quest.

Is the...arrow not appearing over her? >.>

Apparently sometimes the arrow won't be there to point you in the correct direction on this particular quest. At least that's what I heard anyway. I found her on accident before I even tried doing the quest. I was looking around the Bee and Barb for their business ledger to do the Numbers Job for Delvin when I found Maven up stairs hanging out. So I don't know if the arrow would have even been there for me or not.