BOSCH PDO 6 - Cable / Metal Detector

Generally happy with this but struggle on new external walls (we've had an extension) where its seems to pick up metal everywhere. Decided to check it on a bit of Celotex that I have lying around and it picks up the silver foil on it. So presumably the PDO 6 is useless on external walls where there's Celotex or similar. Anyone any thoughts on this and whether there is any other kit that that can tune this out because I need to drill external walls occasionally!
Thanks.
F