I planted brussels sprout seedlings acquired from the local garden
centre about 3 weeks ago. They are taking well to the garden, and I
have sprinkled the immediate surroundings with environmentally friendly
anti slug pellets. However, the leaves of two of the seedlings have
been nibbled to the central rib - yet I cannot see any likely guilty
party - caterpillar or other.
What could be eating my sprouts, and what can I do to stop it?

TIA

Cat(h)