Achieving stealth in space is very unlikely. To hide you need something to hide in, and most of space is nothing. Monitoring systems operate over a considerable breadth of the spectrum, including radio waves and infrared. Even if a ship could perfectly match its background colour at one wavelength range it is likely to be plainly visible at others. The temperature difference between a ship and its surroundings is such that it is the equivalent of a lump of coal on a white bedsheet.
Strategies such as directing waste heat in one direction or hiding behind another astral object have only limited success. This only works with respect to a single viewpoint and the solar system contains many Grigori.
The Grigori are small, unmanned surveillance vessels that monitor the movements of objects within the solar system. Typically they travel well away from the space lanes. Some use photon sails for propulsion, others solar-powered ion drives. Grigori are not particularly fast, but they do not need to get anywhere fast. They are designed to operate independently for years at a time.
Each spaceborne nation has launched a number of Grigori and monitors their transmissions. None of these nations are forthcoming about how many they have operational at a given time. There is even a story that China (or some other nation) has a fully-automated factory ship producing Grigori somewhere in the asteroid belt. A small part of the lunatic fringe maintains this ship will consume all of the asteroid belt within a century or so!
Grigori cannot hide in space any more than any other vessel. What protects the Grigori is their remoteness. To get within weapon range of a Grigori a ship might have to travel days or weeks. Even if unmanned vessels such as AKVs are used this is a poor use of resources. For each Grigori destroyed there are a dozen more observing and reporting the location of the aggressor.