Acuba leaf

Has it been accidentally sprayed or been splashed by any kind of weedkiller?

No hav'nt used weedkiller. Odd leaf was like this, but now quite a few gone brown

Some of the leaves on mine drop off - I'm always having to tidy them up. They are a slightly brown-ish colour, too, but the shrub itself is very healthy. Does your Aucuba seem to be OK apart from this?

Wait and see if any other leaves do the same. It might just be natural.

is a new leaf caught by the frost?

This is exactly what my neighbour's Aucuba was like. I pruned it right back, removing every vestige of dead leaf, as recommended by Bamboo......and it seems to be growing well again! I have a few on mine...and I actually thought it was severe frost damage from the past winter. All the branches were blackened too...on both shrubs ...hence the severe haircut!