Currently, Voyager 1 is floating through the emptiness of space. Does it do anything to indicate its presence, like a light source onboard or broadcasting a signal that makes it more observable to others than just any inert object flying through space?
Any object in space has several sensor signatures:
- Reflected optical - light reflected from surfaces. If you illuminate Voyager, its glint will be detectable.
- Emitted optical - light emitted by bulbs aboard. For energy conservation reasons there are none.
- If there is an insanely sensitive optical sensor exactly on the line between a star and Voyager, occultation detection may be possible.
- Infrared - due to the work of RTGs and equipment onboard Voyager dumps heat into the environment by radiation. At mission start, the heat loss from the MHW-RTGs was estimated at 200 W.
- Radar - Voyager was not optimized for stealth, with lots of sharp corners and no radar-absorbents in use, so its radar cross-section must be quite high (NASA should know the numbers, actually).
- Radiotransmissions - Voyager is still communicating with the DSN back here on Earth. Depending on range and aspect, one can detect the mainlobe, side- and backlobes from this particular source. Other equipment on board may also have detectable side emissions - if you get sufficiently close, you'll spot them. Note: X-band (8.4GHz) transmission comes through two TWT amplifiers of 12.5 and 21.6 W; the antenna dish is 3.65 m in diameter.
- Radioactivity from the RTGs - only when you come VERY close for inspection (Pu-238 emits alpha-particles, which get caught by the berillium shell and transformed into free neutrons which decay into protons).
- Kinetic signature - you'll know it when your craft is hit by a Terran probe.
Now let's suppose you've succeeded in detecting something in the vast expanses of space. My bets are on optics and passive radiowave detection, although I don't know who you are and what detectors you can choose from.
How does one tell an artificial object from a natural one using currently available technologies?
- Rotation. Voyagers are three-axis stabilized, which is sufficiently unusual for rocks to warrant investigation.
- Active radio transmissions.
- Infrared signature. Temperature above the background of deep space for such a small object in the middle of nowhere cannot come from tidal heating or other natural sources.
- ...and, last, when you are close enough, you will see a complex metallic/composite object. No chance for something like that to emerge naturally.
- Raymond L. Heacock. The Voyager Spacecraft. Proceedings of the Institution of Mechanical Engineers. V.194, no.28. 1980.
Thanks to TildalWave's comment, we have an interesting reminder of human imperfection:
Rosetta probe was misidentified for a near-Earth asteroid "about 20 meters in diameter" and designated 2007 VN84. Its trajectory was calculated to pass Earth closer than 6.000 km, which caused a bit of concern that it will impact Earth and even an alert was issued, until its trajectory was later recognized to match that of the Rosetta spacecraft. Just a short anecdote, I thought it's worth mentioning here. Here's a relevant article from the New Scientist (archived).