In the realm of hard sci-fi the detectability of an exhaust plume depends on a variety of factors including the ionization, temperature, shape, velocity, and angle of the plume wrt adversary, and the local medium.

There are a number of methods one could theoretically use to minimize these factors. As an example, imagine the detectability of a small angle coherent neutron beam or the IR exhaust of a black hole starship.

https://en.wikipedia.org/wiki/Black_hole_starship