Because of the longer exposure times used by the Hyper Suprime-Cam survey, the asteroids look bean-shaped or elongated in the HSC search. This trailing is due to the asteroid’s relative motion to the Earth. We’re seeing it move and therefore its light is deposited in slightly different positions along the camera’s field-of-view during the exposure, creating a streak or trail. As far as we can find in the scientific literature, nearly all main-belt comets were detected in images will little trailing. We found one example, that we wanted to share with you. Below is a well-known image of Main Belt Comet 107P/Wilson-Harrington. This observation was actually taken back in 1949 (on glass plates!) by the 48-inch Oschin Schmidt telescope on Mt. Palomar for use in the Palomar Observatory Sky Survey (Fun Fact – I used the same telescope for my thesis survey for distant Kuiper belt objects nearly 60 years later).
Yanga Fernandez, Lucy McFadden, Carey Lisse, Eleanor Helin, and Alan Chamberlin went back to these observations in 1996/1997 and found that this asteroid was active as an MBC with a visible tail! You can find their paper here (unfortunately it’s behind a paywall, but you can read the paper abstract for free). The asteroid is very streaked, a bit more than what you typically see on the Comet Hunters HSC search, but it gives you an idea. The tail is faint and diffuse, but visible off to the left of the streak.
In case you need help spotting the tail, I’ve annotated this version below with magenta arrows pointing to the tail.