I clicked on a few rares on the street today, and saw that the "pick up" option was grayed out, even though nobody was standing near them. When I moused over it, I got the message in the title. I dropped my own collected rares all around Ur the other day, and now I'm worried that nobody will be able to pick them up :( And I can't retrieve them myself because I picked completely random streets to drop them on, and I have no idea which streets they were...
(Just in case the right person is reading this: did anyone happen to see Antique Spigot #150 anywhere? If so, were you able to pick it up?)