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In Nemesis, the Enterprise E is lured to Kolarin III by a positronic signature, which ends up being B-4, another Soongian android.
Would starships of that time period (circa 2379) normally monitor for positronic signatures? Are sensors able to pick up a such a signature from several systems away? (Equivalently, could the Enterprise detect Data, if he happened to be several systems away?)
I'll bite.
I'll answer two and three first. The answer to two is a definite yes. Of course, since they did pick B4 up, therefore they can. The answer to three is probably yes. It's possible Data has some special shielding to stop him being picked up, but there's no reason to think so based on available evidence.
The answer to one is more fun. Positrons are pretty exotic. They're only used in a limited number of technologies. None are mainstream, so all are significant. I imagine normal scans will include positrons just because they're significant whenever you find them and, upon detecting them, sensors will beep to let you know. Plus, as Richard commented, Geordi's visor can pick them up. Since that's basically a sight-aid, complex ship-board sensors meant to do detailed scans of anomalies will be even more attuned to exotic particles.
There's nothing to indicate they were doing anything out of the ordinary with their sensors. However, we don't know that for sure. Clearly they are able to pick up positronic technologies from that range, but whether it's usual for them to try is a question I can't answer for definite. I would assume so, and there's good reason to think so.
Ship sensors monitor for every kind of signal known. Ship senors are primarily passive detection devices, so they are simply waiting for the blip to appear on the scope, identify it, and measure it.
I think the question raised here about the effective range of sensors over light years is a compelling one. In Star Trek, speed, distant, and time were never monitored carefully to create anything resembling a science fiction continuity.
Distant in Star Trek is relative to plot. This means that the story is written and the details are filled in; they are not driving the story. And this is different from almost all other science fiction, driving we the fans a little squirrely.
Implied in your question is the notion that "isn't it kind of a little ridiculously lucky that they found this guy buried in a desert with their sensors and it just so happened to be another Soong type Android?"
I think the writers intended you to be skeptical. As I recall, B-4 was placed their by Shizon to lure Captain Picard. And so, it was ridiculously lucky and you are right to be skeptical. (Except, it wasn't actually lucky at all -- It was a trap!)
However, on the flip side of the coin, as PointlessSpike mentioned, positrons are not naturally occurring. In Star Trek, rare things are always easier to scan for. (This seems to be a truth in regular life as well. The person acting strangely always stands out in a crowd. We notice the one woman wearing bright red lipstick.) Therefore, positrons are discernible on Star Trek sensors at "great distances." But remember, distance is relative to plot. Frankly, I think several star systems away (easily 15-50 light years) strains credulity even if distance is relative to plot. I can't think of one instance in Star Trek where such fine detail (such as the location of a head) is discernible over a distance greater than the distance from orbitting Starship to a planet.
It would be far more believable to find the android floating in deep space. In this case, there would be no noise and interference from THE PLANET. The ship could have followed faint positron emissions like bread crumbs through space until the android was found.
But at the end of the day, it is subjective.
