'Pepper' has fully articulated arms and hands but moves about on wheels HIDDEN under a skirt.

It is a grammar question, please tell me why we use HIDDEN and not ARE HIDDEN.

See a translation

@VictoriaFrench

It is quite normal to collapse, or contract, this type of sentence from its full version – which would actually be –

… moves about on wheels WHICH are hidden under the skirt.

If it is possible to drop some words, while maintaining 100% of the meaning without any confusion, we will very often drop those words.

The table has a strong bar placed between the legs – Technically, this sentence should be –

The table has a strong bar which is placed between the legs

– but it is impossible to read the contracted sentence and not correctly understand the meaning. The correct meaning is 100% obvious and easy to digest.

And that is why, once you start looking, you will find these types of sentences everywhere. If the words are not strictly necessary, and the meaning is not affected, the words will probably be dropped.