In my book, Oumuamua was actually a large alien invasion/colony vessel.
So, suppose we have an invading ~100 km long alien colony ship flying towards Earth. Suppose also that the aliens are unable to fully cloak themselves, but have a way of altering/reducing their visual or infrared signature.
Is there any reasonable way they could trick Earth telescopes (like Spitzer) into believing they are registering a much smaller, Oumuamua-like asteroid? The aliens probably shouldn't worry about fully mimicking the exterior of the asteroid, as Earth telescopes would only see a pale dot.
The aliens are carbon-based and 3-dimensional. They are not familiar with space-time warp (the ship was travelling at subliminal speed) and do not posess other hyper-advanced tech. So, they can for example have beam weapons, but are unable to phase out from reality or perform any other hard to understand tricks which we know nothing about. Their technology needs to be at least somewhat understandable.
How would they go about that?