As Fraser says.
It wasn't so long ago that camo netting only had to defeat the Mk 1 Eyeball. Then it became necessary to ensure it didn't show up unnaturally bright or dark in near-IR. Then along comes LWIR, changing the goalposts again. Add in other spectra and suddenly it becomes fiendishly difficult to fabricate a camo net that doesn't act as a dead giveaway at some wavelength or another.
The same is potentially true of everything an adversary may have. If there is, say, an item that shows slightly in each of LWIR, MWIR and SWIR bands then a moderately intelligent can look for a match on all three together and declare it a threat with much more certainty than would be the case for one channel alone.
The more technically sophisticated side has the advantage of being able to engineer its materiel to minimise its spectral signature now, thus obtaining a significant and long-lasting advantage over its enemies who would not only need to deploy new, low-signature kit, but in order to have any sort of chance they would also need to develop sensor technology that leapfrogs their opponents, who have already sorted out everything to meet the current state of the art.
Fraser mentions VLIR; I suspect it won't be long before deployable 'terahertz-gap' sensors are developed, meaning that nothing in the spectrum from radio to ultraviolet is 'safe'.
Superman's X-Ray Vision is probably just around the corner.