I also have a bit of a mystery, at least to me. One of STR's renderings was reposted in the Spire thread, and I noticed a building which has been in his renderings the whole time (it's in the aerial I have as my wallpaper), but I didn't notice until this angle.

(rendering by STR)
What is the building I circled? It looks to be about 800ft, but it doesn't match any building or proposal I know of. It's not 29-39 S LaSalle. From the aerial render, STR has it at Clark and Monroe.
Was this some old proposal that got canceled?