No way. The typical density of gas in one of those pretty nebulae you see pictures of in Star Trek episodes or other science fiction movies is only at most a few hundred atoms per cubic centimeter. This makes even the interior of a dense nebula a better vacuum that we can easily produce under laboratory conditions.

If your sensors used light, there would be no problem seeing them to a distance limited by the resolution of your optical sensor/telescope. If you were using radio waves, however, the interior of a nebula can contain ionized gas for many cubic light years. Ionized gas at these densities would become a good cloak to a tiny star ship if the sensor was operating at one of the frequencies 'mirrored' by the plasma in the nebula. This is the same principle that lets the ionosphere of the Earth reflect low frequency radio waves, but transmits high frequency radio waves. Still, if you were less than a few million miles from the ship, it would be visible at all frequencies because the amount of ionized gas between you and it is still so low that the medium should be pretty transparent even at radio frequencies.

Perhaps the only region where a spaceship would be able to hide is in the accretion disk of a forming planetary system or one around a black hole. But the meteorite and radiation damage you woudl receive would easily be more of a concern to your safety than anything you could possibly be hiding from!

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