At night, in a large body of water, every vessel appears to be either
at anchor (minus any type of speed boat or ferry). This is especially
true if the boat is moving directly away from you or towards you.
MARPA capabilities on RADAR help with this somewhat.

The AIS ability to see ALL vesels and know with certainty if they are
at anchor or steaming, and at what heading and speed takes much of the
guess work out of the equation of safety.