In this video at 7:12 you can see a thermal shot of a landed space shuttle. There is a periodic white cloud coming out of the top. What is this cloud?
Supplementary answer because I like pictures.
Here's a schematic of the APU system. You can see the 3 exhaust ducts at the top, two on one side of where the vertical tail goes, one on the other. This is from the 1982 Shuttle Press Reference - so realize that it's not what the system looked like at the end of the program because the APU system underwent some extensive redesign. But this is still an accurate depiction of the exhaust ducts and vents.
Here's a top view of Discovery approaching the ISS on STS-121.
I cropped the picture and indicated the 3 vents.
You can see an ignited APU exhaust plume and the characteristic pulsing of the APU exhaust (it used a bang/bang speed control scheme) at about 4:49 into this video of the STS-134 landing.
That is the exhaust of one of the Shuttle's three APUs. These hydrazine powered turbines provide power to Shuttle hydraulic systems.