I've been doing some more investigation of the apparent anomaly reported on a number of occasions whereby SIN local traffic doesn't appear to plot reliably. It occurred to me that, while it's possible on the network to watch arriving and departing traffic at many of the world's airports, I've never seen anything within 100 miles of SIN.
Following up my theory that proximity to the equator might have something to do with it, I've been analysing network traffic, with interesting results. It would appear that SIN arrivals from Europe, India, China, etc, are trackable until they get to 2°N, at which point they stop tracking. Similarly, departing traffic on these routes is invisible until it passes the same latitude northbound.
Next step was to look at South American equatorial coverage. We know Brazil has a good RadarBox network, so analysing traffic between Brazil and North America/Europe seemed a good idea. This time, there seems to be an invisible demarcation line at 2°S, after which southbound traffic suddenly appears and northbounds disappear.
Of course both of these phenomena could also be explained by lack of user coverage, so I'm not suggesting that the results are conclusive at this stage.
If anyone else would like to help out, I'm looking for evidence of aircraft tracking in the zone between 2°N and 2°S, anywhere in the world. My own RB is currently set up to generate alerts for flights withing 120nm (2°) of 0N 104.5E (i.e. the point on the Equator closest to SIN). No alerts so far - but then it's the middle of the night there at the moment so we'll see what tomorrow brings.
When reporting presence or absence of traffic in the twilight zone, details of which RadarBox version you're running would also be useful.