If Planet Nine indeed exists, how large a impact event there should be to be seen from Earth?
We can compare the brightness of a comet hitting planet 9 with the 1994 impact of Shoemaker-Levy 9 / SL9 on Jupiter. It was seen by Hubble, and Galileo which was orbiting Jupiter at the time.
View from Galileo of SL9-W, a 49 second event involving a 350m comet fragment at 215,000km/h:

info from NASA:

_Galileo RADIOMETER

brightness of L-impact &gt; that of G-impact by 20 percent   * G impact about 15 percent of total light from Jupiter at
maximum



_CCD (late 1980's tech, 800x800) time resolution 2.5 sec

data returned for K, N, V and W impacts   * W-impact clearly seen as light point with 1 percent of total luminosity of jupiter, and afterwards the fireball  roughly constant during 49 secs at 15 percent of total Jupiter intensity.

_UV Spectrometer at 292 nm

G-impact seen at 07:33:32; 20 percent increase in total
Jovian brightness   * secondary impact at 07:34:36, 1/4 as intensive as G
itself


The fireball events from SL9 reached 3200km above the cloud deck the total energy released was 60,000,000 Megatons of TNT.
Jupiter is 5 Astronomical Units away and Planet 9 is estimated at ≈500AU away, so an SL9 equivalent impact would be 100 times further away and 100x100 times less bright than SL9 because of the inverse square law, corresponding to 13.6 apparent magnitude difference compared to SL9, which was 20% of Jupiter's brightness.
Perhaps i'm wrong, i'm suggesting a maximum visual magnitude of 13.4 / 11.2 for an SL9 event of comets ranging from 350m / 3.5km for planet 9.
Asteroids are probably very rare at 400AU, otherwise a processor could check historical records for matching flashes of light where Planet 9 would emit single or multiple flashes ranging from 5 to 20 apparent magnitude, conforming to a characteristic signal of a 5-10 second rise, a 30 to 120 second constant brightness and a fade.