If Planet Nine indeed exists, how large a impact event there should be to be seen from Earth? Recently I've found this news article, A meteorite crashed into the Moon during total lunar eclipse in January, about a collision into the moon that happened while a lunar eclipse was under way. According to a paper{1}, it produced a flash that peaked about 4.2 magnitude, what puts it in the range visible to the naked eye, assuming a threshold around magnitude 5:

Despite visible from Earth, it was caused by a pretty small object, about 45kg, with impact energy about 1.5 ton TNT equivalent:

That made me wonder, if we were lucky enough, could we pinpoint Planet Nine position just by looking in its general direction and waiting for a large enough impact with a KBO, able to produce a visible flash?
Or instead of a visible flash, perhaps we could at least hope to detect the infrared glow from the heat produced. For example, assuming a Planet Nine about twice Earth radius at 800 AU, I estimate the amount of energy received from the sun in a Earth day to be about $10^{17}J$:
Python 3.8.5 (default, Sep  4 2020, 07:30:14) 
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type &quot;help&quot;, &quot;copyright&quot;, &quot;credits&quot; or &quot;license&quot; for more information.
&gt;&gt;&gt; solar_constant_earth = 1362
&gt;&gt;&gt; solar_constant_9th = solar_constant_earth * (1/800)**2
&gt;&gt;&gt; solar_constant_9th
0.0021281250000000002
&gt;&gt;&gt; 9th_cross_section = 3.1415 * (2 * 6350000)**2
  File &quot;&lt;stdin&gt;&quot;, line 1
    9th_cross_section = 3.1415 * (2 * 6350000)**2
     ^
SyntaxError: invalid syntax
&gt;&gt;&gt; cross_section_9th = 3.1415 * (2 * 6350000)**2
&gt;&gt;&gt; daily_energy_input_9th = solar_constant_9th * cross_section_9th * 60 * 60 * 24
&gt;&gt;&gt; daily_energy_input_9th
9.316555641045002e+16

This is about the estimated yield from the Tunguska event, so I think even a impactor as small as 100m across delivers the equivalent to a day worth of 9th total solar irradiation in a split second, so the infrared glow of the planet could be boosted a lot, briefly.
{1}. Madiedo, José M., et al. “Multiwavelength observations of a bright impact flash during the 2019 January total lunar eclipse”. Monthly Notices of the Royal Astronomical Society, vol. 486, no 3, julho de 2019, p. 3380–87. Silverchair, doi:10.1093/mnras/stz932. link
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.