It wouldn't make sense for others to be roaming around with Janus Devices. The only reason there's one working at all is because you delved into Alpha Site and lived to tell the tale, while grabbing valuable data from the Ziggurat's motes. The reason there's not more devices or further research on opening the Gate Network proper is because the engineering team responsible for developing your prototype (which, by the way, you brought together to begin with) jumped ship immediately after finishing it, taking their necessary data and expertise with them. Hinting that someone has another Janus Device would add plotholes, not fill them.
The Janus Device doesn't even restore the Gate network, it just allows temporary traveling between scanned gates, at high fuel costs. A properly restored Gate network would allow for instantaneous, free (at least for the traveler) transport between any gate in existence.
The car analogy doesn't work: your "vehicle" is your Ships, and nobody's forgotten how to use them or even build them. The gate system in its heyday is analogous to a currently-defunct theoretical magical superhighway infrastructure that used to allow for instantaneous, free travel between two points. The Janus device equivalent would allow for a small sliver of the Superhighway infrastructure to be placed back online temporarily for instantaneous transport, but its usage demands energy (fuel) and a prior visitation to both points that you want to travel between; it's no replacement for the Superhighway in its prime.