99.99% probability is the smoke is coming form a leaking seal on the evaporation chamber, which sits on top of the Valve cover.

It is a silcon (paste) gasket and a common failure point on these engines.

It is NOT serious and can be replaced in about 30 minutes per side (+24 hours for the sealant to cure).

If it's not too late I would warn whoever is doing the repairs that the small aluminium torx bolts WILL snap if any pressure is put on them - and worth checking that they are intact on return.

Check the heated seats too!

Enjoy the Xfire... I love mine!