Some minutiae:
I dunno, I would think activeness would make it harder. Something about not being in the same place twice.
The point would be that an active ship would emit stuff: warp fields, unusual concentrations of neutrinos, whatever. If there were no emissions, the ship would essentially be a miniature asteroid, a dead rock that would only register if you bombarded it with the emissions of a radar-like device; if there were emissions, passive sensors would have a chance of picking up the ship against the inert background of space.
Essentially, the difference would be like between an extinguished torch in the night, and a lit one... A good pair of binoculars would let you spot the torch from a considerable distance if lit, but you might pass it by two centimeters otherwise and not notice a thing. Easily a millionfold difference.
And you didn't comment on the dyson sphere. It's still a valid argument considereing they practically tripped over it at a full run.
Perhaps; but the fact that you might notice a structure the size of a solar system doesn't exactly translate to your spotting structures the size of starships at any regularity.
And speaking of regularity, the
Jenolan was supposedly on a milk run to a well-established colony, apparently a popular retirement spot for weird-accented engineers, when she was forced out of warp by a rare malfunction and spotted the sphere. Apparently, none of the previous, non-malfunctioning Federation travelers of the route had made the discovery...
The fact that the star's radiation was masked by a giant veil was apparently enough to make it unobservable; the gravitic pull was the first telltale that the
Jenolan or the E-D observed, although I'm sure other things could have been seen after they knew where to look. When a largely non-radiating object the size of a star system can hide from Starfleet along a frequented spacelane despite having massive gravitic pull and possible thermal exhaust of some sort, the odds of anybody stumbling onto an even less radiating object a trillionth of the size begin to seem dim.
Still, some relevant examples of Starfleet stumbling onto dead spacecraft:
In TOS "Space Seed", the
Botany Bay was essentially dead. She had a radio beacon going, but it seems that Spock's "sensor readings" were what lured our heroes there in the first place, and Uhura picked up the beacon only after the ship was in the crosshairs. Passersby with 23rd century Starfleet sensors and Spocklike skills
should thus have been able to pick up the
Stargazer, too.
In TNG "The Neutral Zone", the cryosatellite was essentially dead. However, she was within a solar system where our heroes were already present and idly scanning the surroundings. The proximity would improve the odds a lot - but then again, if the
Stargazer remained in the Maxia Zeta system, passersby would enjoy proximity, too.
Timo Saloniemi