Observing a Magnitude 21,2 asteroid?

When testing out the new ASI183MMPro CMOS camera, I pictured the galaxy NGC3628. This is a well-known galaxy, sometime called the ‘Hamburger’ galaxy, part of the Leo triplet.

When processing the image, I noticed this very faint trace abit to the south of the galaxy. I used the Minor Planet Checker to verify any known asteroids in this area, and I found only one a bith south of the very little galaxy underneath NGC3628 that I used the coordinates from (using SIMBAD and ALADIN).

I doubt the magnitude of 21,2. When I compare to a star nearby that has the same weight on my picture as the asteroid, that star is identified by the GAIA catalogue as magnitude 19,5. So my estimate for the asteroide, since it’s a moving object would be around 20.