The universe is a huge place that is full of planetary systems whose extension is also very large. The number of possible locations in which an unknown fleet or base can be found is almost infinite. Nevertheless, it is possible to equip our bases and ships with sensors that sweep all the space of the system in which they are to locate unknown objects.
The possibility of finding an unknown object in a system depends on:
It is the detection force of the object you are looking for. It depends on the quantity and quality of your sensors.
The presence is the measure with which a space unit (ship or base) passes or not unnoticed before detectors of other factions. The higher the presence value, the more attention is drawn to one unit, which increases its chances of being detected. Large objects have higher presence values, but the structure of these can reduce this value. Camouflage generators can also attenuate their signal.
The greater the distance, the less chance of detection.
Mechanics of detection
On a regular basis the units (fleets and bases) scan for unknown objects. Detection of a particularly difficult to locate unit (for example a small, distant object) is likely to occur after many hours or days and many sweeps of the system. It should be noted that the fact that we are not able to detect an object does not mean that it is not there.
In general, larger objects such as bases or large fleets will have a greater presence and will be located faster. If you want to go unnoticed, it will be best to use small fleets with small ships or bases that are designed to be hidden.
Fleets disappear from the detection list once they have left the system where they were detected. The bases never disappear from the detection list because they can not change their position.