This should seem obvious, but an impostor needs to avoid being seen when they are skulking around killing people. One way impostor players often get caught is by being witnessed killing. But they are not seen by a nearby player. Instead, they are seen killing on a security camera. It’s a small thing to miss and is easily overlooked, but there are cameras scattered throughout the map. If one is flashing red, that means someone is watching the feed from the security room. So always be sure to check the camera’s status before eliminating another player.
Just as important is to never be seen using the vent system. Ever. Traveling by vent is an impostor-only ability, which means if an impostor player is seen using it then they more or less just gave up their identity. So it is advised impostors avoid using the vents too often—especially toward the beginning of a match when there are more players around.
In fact, some players may only want to use the vents for quick escapes after committing a murder. Getting caught near the body will only seem suspicious after all, so going someplace that is not the scene of the crime seems reasonable. Just be sure to exit into a room where no one is present. Avoid using the vent system to enter a high-traffic or open areas like the café or the administration room. Also, be sure to avoid using the vent system to enter areas that have an active camera in them.