How close to the Ringworld was the Fleet of Worlds?
At the time of the first expedition (2851), the Fleet of Worlds was a little more than two light-years from the Ringworld. This is given on chapter 7 ("Stepping Discs"):
They were in hyperdrive for a week,
covering a little more than two light
years. When they dropped back into
Einsteinian space they were within the
system of the ringed G2 star; and the
foreboding was still with Louis Wu.
The Fringe War takes place forty-one years later, by 2892. The Fleet of Worlds had been accelerating steadily for more than 200 years at the time of the first expedition. According to chapter 8 ("Ringworld") of the first novel it was already at near-lightspeed at that time:
The puppeteer worlds had been moving
at nearly lightspeed along galactic
Having travelled at near-lightspeed for forty-one years, the Fleet should be somewhere between about 39 to 43 light-years away from the Ringworld when the Fringe War takes place.
Would the Fleet be detectable by the participants in the Fringe War?
I think not. The Fleet has no sun, and the artificial suns that are used in its place are not very bright because they're very close to their worlds. We first see the Fleet on chapter 5 ("Rosette"), and we're given the following description:
Five dim stars, in a regular pentagon.
They were a fifth of a light year
distant and quite invisible to the
naked eye. At present scope
magnification they would have to be
full sized planets. In the scope
screen one was faintly less blue,
faintly dimmer than the others.
At a fifth of a light-year, the Fleet cannot be seen with the naked eye, and Louis Wu's scope can only resolve them to what looks like five dim stars. The participants in the Fringe War would be receiving the light sent when the Fleet was approximately twenty light-years away. That is one hundred times farther away, which should make the Fleet pretty much undetectable.