They aliens' arrival emit gravitational waves when leaving FTL. LIGO detected it.
The alien fleet's FTL technology emits a burst of gravitational waves when they leave supra-luminal speeds.
IRL, gravitational waves are only detected when cosmic events in the scale of black holes fusing or supernova explosions, but these are thousands of light-years away.
Assuming that gravitational waves follow the square-distance law ref, we can compare the required energy/mass in proportion to the square of the distance to "beyond pluto's orbit" (6.25087891e-7 LY^2) to the square of the furthest black (5.29e+18 LY^2).
The magnitude of the gravity waves produced by the aliens need to be around 10-25 times the waves produced by the black hole merger.
A typical stellar-class of black hole has a mass between about 3 and 10 solar masses. ref
So your alien fleet doesn't even have to be massive. A solar mass is about 1.989e+30 kilograms. Multiplying all that, the fleet just needs to be about the size of an oil tanker ship. An invasion fleet should be way bigger than that.
Tankers move approximately 2.0 billion metric tons ref
The only thing your FTL technology really needs is to emit gravitational waves. The rest is covered.
LIGO can pinpoint direction, so scientists will know exactly where to look for, and maybe spot your aliens within your timeframe.