If they can miniaturize the re-entry vehicles enough to make them undetectable, or if they can leave them "parked" next to larger satellites so as to hide them,

If they can miniaturize the re-entry vehicles enough to make them undetectable, or if they can leave them "parked" next to larger satellites so as to hide them, then having such hidden orbiting warheads would seem to allow for plausible deniability in small nuclear bombings. But since everyone would know only we have this capability, the deniability goes out the window. Also, re-entry vehicles get very hot and are visible, and their trajectories could get plotted, thus providing evidence of whodunit. Plus the x-37b can't possibly carry very many warheads and re-entry vehicles. On the whole I think it's very unlikely that the x-37b is placing nukes in orbit.