>> What kind of birds are cold in black-hot imagery? What sort of missiles don't have an exhaust but a "ghost shell" trailing behind?

IR imagery can be flipped between black=hot or white=hot. These systems are about creating contrast to aid visualization, not recording scientific data.

>> What sort of balloons show up as contrast instead of neutral?

A hot air balloon? Any balloon that has recently changed altitude? Any reflective balloon reflecting sunlight (Mylar is common). Or, in thin air, a non-reflective balloon absorbing sunlight and warming faster than it can dissipate that heat.