AA wrote:

I was surprised - is this common in dometic buildings?

Reasonably so I think, although I've never come across it or used it
personally.
(this is a brand
new house by a relatively good quality builder). My main question is
whether it is safe to drill into these for support when fixinng
mirrors, shelves etc or is there a danger that they are also conduits
for pipes and or cables.

Difficult one: they *should* be just metal studs, and there *shouldn't*
be any cables in there buried in there unmarked, outwith the specified
permitted zones. But I certainly wouldn't feel comfortable drilling
into them blindly.

I'm not sure if
I would trust the voltage
setting on my detector to find wires behind metal.

No, neither would I!

Can you attach your mirror to the wall using plasterboard fixings (if
it's not massively heavy) or mirror adhesive? Sounds a lot less iffy to me.

David