I've initiated the quest to access the Thieves Guild.
Brynjolf provided me with a distraction so I could loot the ring and plant it on the target. He told me to meet him in the Ragged Flagon in the Ratways. After waiting for some time, he never showed up. I checked his stall and he hadn't moved at all. A large group of people are still surrounding him like he's still providing the distraction.
I've tried waiting by his stall to see if he will move, but he doesn't budge. I've left town, entered and exited buildings, ran through the Ratways a few times and nothing seems to be working.
Watch now! The sun is fading away. But still, the skyline is glowing bright white.
He's doing the exact same thing for me, one time I sat next to him in the market for a minute or two and he started heading down there. After that he went around and activated the lever, and walked right back outside. I still haven't found out what he's doing, but now he just wanders around Riften doing nothing.