Somebody's been reading "Rendezvous with Rama"...

I think the easiest and most obvious attribute of such a vehicle would be its astonishingly high porosity. Spacecraft are essentially bubbles of skin material, enclosing a habitable volume.

Long after the radiation has decayed and the surface has been eroded and disguised, the giveaway would be an almost total lack of mass.

Isn't there a metallic asteroid with 80% porosity?

- 16 Psyche: 250 km in diameter, metallic, and mean density = 1.2, rather than the expected 7.5 for Ni/Fe - that's 84% porosity!!!

So it could be a hollow sphere with walls ~14 km thick, or a former 3D swimming pool for a race of spacegoing dolphins, with walls only 2.25 km thick. In the latter case, it would be deep frozen.

(Or the measurements of its mass could be way off!!!)