When there's not enough room in a zoned area to put up a building, it gets filled with a random piece of "junk" instead. This is most obvious in industrial zones because they tend to have larger space requirements, but it happens in residential areas too.
Sometimes I see junk appear in areas that actually can hold buildings, as proven after I delete the junk and a building later appears. Other times, the junk comes back and I have to delete it again a few times before something finally builds. Finally, should a building get bulldozed or replaced, deleting junk seems to have an effect on where any replacement building appears.
Does the game ever proactively delete junk when it comes to constructing new buildings, or do I have to constantly delete it myself?