I am looking for information on this quest. I do know where to find her - every Wiki will tell you.

I do remember from the Thieves Guild that one of their members (Tonja, Vex or Sapphire?) had some additional information on her, however, the dialogue has disappeared and it did not get logged by the journal. I tried rolling back my save files, but I cannot find this dialogue anymore. It seemed to create a trail that the player has to follow. Since the journal did not track this bit of information (thanks for that...) and I had more important things to do did I forget about it.

Can anyone remember what this bit of information on Svidi was?

Does this help?

http://elderscrolls.wikia.com/wiki/Svidi

No.

Sibbi Black-Briar tells you about her and asks you to find her for you. He does however not give you any clues where to look. So you end up asking around in town and I could find someone who had known her. Apparently did she have connections to the Thieves Guild and one of their members tells you a bit about her.

I do not think that the quest objective is to run through all of Skyrim and to ask every NPC about her and in the end to find her only by chance. It would be a dumb quest design, and since one can find a lead within Riften would I like to go after it and see if there is a trail one can follow.

The quest journal does not track this bit of information and gives you no marker, but it lets you locate the quest on the map and gives you random locations. I am not sure what to make of it. First Bethesda dumbs down the game with an over-helpful journal and then they put a quest into it that lets you run around dumb and stupid searching for clues...

It is only a side quest, but still, I do not want to put it down as a dumb or a broken quest. I'd appreciate it if anyone can remember the lead given by the Thieves Guild.

Dumb chance is how I found her.
I couldn't found anyone who I could ask about her so I forgot it. Then at a later stage I talked to a girl in a tavern and out of the blue one of my first dialogue options was to ask if she was really Svidi.

lol, yeah. They should have made it so that every female NPC in Skyrim has this dialogue option.

"You are Svidi, aren't you?"
...

Then, once you asked 3 or more if they were Svidi, have some of them come forward and say:

"I am Svidi!",
"No, I am Svidi!",
"Take me, I am Svidi!",
"It is me. I am the real Svidi, the one and only.", ...

... and then you get to watch a girl fight!

I gave you a link which answered your question and told you where to find the girl, but you say that wasn't helpful. No pleasing some people.

And I wrote that I already knew about it. You just cannot read, can you? If you want to please me then start there.