Clues

(i) The Nomads have lost ten ships in five years.
(ii) All disappeared in the direction of Sagitarii.
(iii) No mechanical cultures have been found in that region.
(iv) Otherlings have also lost ships in that region.
(v) Coordination Service and Galactic Survey have not.
(vi) There are many apparently uninhabited Earth-like planets.
(vii) Natives on some inhabited E-planets seemed familiar with spaceships -
(viii) - but had never before been visited from the Stellar Union.
(ix) There were surprisingly similar plants on some supposedly uninhabited E-planets.
(x) Galactic Survey had found a dozen identical plants on six uninhabited planets.
(xi) Tiunrans, from an M-planet beyond Vega, passed through that region four centuries previously but had not transplanted plants and had lost four ships.

Nomads and Cordies investigate.