Eh, I am not sure which area you are speaking of, in my experience the chests and barrels near the basement of the Smugglers Den are surrounded by Smugglers that have mage support, that will cast see invis.
There is a stray pile here or there, but nearly everything down there has tremorsensed TS. Or can be considered a bit of a risk with Gangleaders and Wererat Lords or Abbherant Bugs.
It may be a tad over-rewarding but you make it seem like its off the charts and I caution to not go overboard. Good catch though, I just was down there yesterday and noticed some of what you speak of.