Any ideas on what could be eating my coriander plant?  I planted two small to medium plants in the garden last week and now all the leaves have vanished and I'm left with just stalks.  There are no visible slug or snail trails (not even sure if they enjoy eating coriander).  All my other herbs are intact.

I would expect this to be slugs or snails.
They don't often leave trails and will usually eat anything when it is still small.
I usually grow items until they get to a reasonable size and then plant them out. They will hopefully have become unpalatable or too tough by then.