You seem to have a pretty good intuitive grasp of astronomical technique, particularly since you haven't been trained in it. Astronomy is driven very much by our ability to observe phenomena, not so much the phenomenon itself.

The infrared detectors on this satellite operate on IR wavelengths emitted by very cold materials, such as gases in deep space, or asteroids in our own solar system. Any anomalous source much brighter than its immediate surroudings would definitely be worth investigating. IR also penetrates the interstellar medium very easily, but not our atmosphere, which is why we rely on satellites for it. The "negative spike" you mention sounds a lot like an "absorption line" spectral feature. They are caused by other processes, but the observing technique might be very similar.

I'm not the first guy to suggest using IR data for SETI. THe only assumption is that ET may have industrial processes that produce enormous IR signatures that would not occur in nature--not an entirely unreasonable one. The Dyson sphere is the best candidate so far, it would be a huge infrared point source that emitted nothing at visual wavelengths. The old IRAS imagery is online, and people are scanning it for "unnatural" signatures too. So far, no luck.

I'm starting to wonder if maybe Fermi was right. Life may be very plentiful. Maybe even intelligent life. But technological life may not. But it's at the top of my bucket list, even if there's nothing I can do to make it happen.