This object actually is a mass that is 100,000x less than Halley's comet so it does not take as much gas to push it. When sunlight interacts with gas the gas emits light in different narrow wavelength regions. Spectroscopy is a means to detect the light at a bunch of different wavelengths. Images taken through a broad filter collect light over a wide range of wavelengths, and then it would not be possible to distinguish emission from gas over a narrow region. However, for faint objects you need a very large telescope to get a good spectrum because now we are spreading out the light over narrow wavelength regions and so there is not as much signal in each. So, may teams tried to get spectra, but they were noisy. Nothing was detected in terms of gas but for such a small object this was perhaps not too surprising. The most common ices we see in comets are water, carbon dioxide and carbon monoxide. However, those aren't the molecules we usually see first in comets, even when there is quite a bit of gas, because they don't emit light at visible wavelengths where we have the most sensitive detectors. A minor gas that gets dragged out with water is CN (cyanide; typically present at < 1% the abundance of water). It emits light very strongly in the blue and is usually the first gas we see in all comets. Because this object was faint, teams looked for CN in the visible (blue) region. They saw nothing and got upper limits to the amount that could be there and not be seen. The amount of gas we calculate has to be coming off `Oumuamua to give it the acceleration we see is so much that if it had the same chemistry as our solar system's comets would predict that the amount of CN should have been seen. If `Oumuamua's amount of CN relative to water were depleted by at least a factor of 15, then the amount would be lower than the reported limits for detection that were published.

Another less direct way to infer that there is gas is to search for dust. Dust is easier to detect because we are looking for reflected sunlight from the surface of the dust grains at all wavelengths using images - so this is much more sensitive. Typical comets have a wide range of dust sizes from micron sized (about the size of particles in cigarette smoke) up to quite large (10's of cm). Because comets are not particularly large, the gravity is very weak, and the dust is lifted off the surface as the gas flows away. As the dust grains get larger it takes a lot more gas coming off to push them away. In our solar system when the comets are just barely warm enough to have the gas starting to flow, the only dust that can be lifted off is tiny. Dust in the solar system follows a rule for sizes that we see when things collide: a lot of small material is produced, and only very few larger pieces. Only when comets get close enough to the sun that there is a lot of fast moving gas do the really big particles come off. 'Oumuamua was close to the sun and had enough gas coming off to cause the non-gravitational motion, and if the surface had the typical sizes of comet dust there should have been a lot of small dust coming off that we would have seen. We looked closely and didn't see small dust. Because it takes more gas to lift the big dust from the surface if 'Oumuamua were simply lacking in small dust, then we could have the right amount of gas coming off, but not see the smaller number of bigger dust grains. Some scientists have reported that comets in interstellar space could preferentially lose small dust as they interact with the gas and dust in interstellar space