Got a better picture than that? You have got to do better than an oblique angle to make your point.
While it is interesting, I still think that the "vents" could not be over 120 degrees, where the exhaust from the engines are seen as a tongue of 400 degree heat over 30 feet long. 4 nice, big, juicy, hot engines and it goes for the A/C vent?
I dunno.....
Mr. Quarterpanel:
An infrared seeker looks at the total amount of IR, and that is not always directly reallotted to temperature.
The amount and spectrum (color) of IR is determined by the "blackbody equation" or Planck Energy Distribution Law (and the Stephan-Boltzman Law)
http://www.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html
There is another factor and that is the "emissivity" of the hot body. Basically this is the amount of IR that the body emits. If I have two pieces of aluminum, one flat black and the other mirror bright, at the same temperature, the flat black one will emit much more IR than the shiny one because it has a higher emissivity. Both will emit the same spectrum of IR, though.
What I'm getting at is the plume of hot air does not necessarily emit a larger quantity of IR than a black surface at a lower temperature, because the emissivity will be lower.
There is a picture posted above that shows internal engine components and the AC vents all glowing a cheery white, and nothing for the air plume, which is, as you remarked, quite warm and large.
http://www.mikroninfrared.com/news/mbrc.htm
Here is an IR calculator (you must register to download) that lets you figure out how much IR is emitted from a surface at a given temperature. You can look up emissivities online, or just pick a temperature and vary the emissivity to see what happens.
Emissivities:
http://www.raytek-northamerica.com/cat.html?cat_id=9.5&PHPSESSID=publicRaytekNorthAmericaIt's also worth noting that a heat-seeking missile does not tend to just chase after any source of heat. Oddly enough, it isn't as easy for a Stinger-type missile to take down a passenger airliner as people might think. These missiles are designed to shoot down military aircraft whose engines tend to burn hotter than even a large passenger jet, so the possibility that a missile would lock on a minor heat source on a 747 seems a little far-fetched to me.