In Star Trek III: The Search For Spock, why didn't Kruge know there were only five people on the Enterprise bridge? Could't the Bird of Prey scan to know how many life signs were aboard the Enterprise? Has there been any in universe answer to this in films books etc?
A real-life modern warship could operate (badly, and for a short time) with a crew of three: one person on the bridge, one person in the combat control center, and one person in engineering. However, were you to come upon a modern destroyer cruising by, checking to see how many crewmembers you could see would likely not be the first thing on your mind because the assumption would be that it's fully crewed, not that there's only three people trying to operate it.
Kruge came across a well-known starship of a known class not knowing that it had been stolen by a handful of people. There's no reason for him not to have simply assumed it was crewed by the normal complement.