Read third note in bugs section
Seems like there is a glitch.
I played this recently and have done it several times on ps3 as well.
Even with Dawnguard dlc which adds random attacks in Riften and other towns.
The good thing about these guild quest lines is that they can be done with a new character to get the achievements. Thiefs guild can be very long one to complete as it was artificially made longer by the developers , so it can be a pain. There are tips here and elsewhere on how to do it quicker.