I don't know what the USVs are made of. If it's fiberglass, they may not provide much of a radar return.

googles

https://christinedemerchant.com/radar.html

Not all substances reflect Radar equally well. Electrical conducting materials such as metals are better than insulators such as wood or fiberglass. Carbon fiber is better than Kevlar or fiberglass. Flat surfaces reflect better than curved surfaces.

Wood and fiberglass boats are not very good radar reflectors and tend to disappear. Even if small craft have materials that should reflect radar, it is often located quite low or even below the water line making it more difficult to see. Motors should reflect but because they are low they tend to not. Round masts even if metal tend to scatter radar signal and not provide good echos. In order to be more visible to radar small boats often install radar reflectors.

They already have a very low profile, need to put their camera on a mast to reliably see the target.