As
@westward10 says, non contact voltage testers can only give an indication, which isn't always accurate unless you interpret them correctly.
That looks suspiciously like a LAP tester, which in my experience are not very good quality either so I wouldn't necessarily trust it, though it makes sense to be sure it is safe.
Is the ceiling light on in the picture, it's hard to tell? If it is, do you get the same reading with the light off?
Was it installed by someone who knew what they were doing? It's possible that it's been installed with reverse polarity, so that the neutral and live are crossed - which could mean it might detect the 'live' through the metal even with the light off - though likely only if the metal is thin, or perhaps not metal at all.
The only immediately dangerous option would be that if the earthed metal had become live, in which case it should be installed in a way that would trip the mcb or RCD in the consumer unit.
Is the lighting circuit on an RCD in the consumer unit/fusebox?
it's hard to diagnose any further without a 2 pole tester used correctly to get an accurate view of the installation. If you're unsure it may well be worth getting an electrician to pop in and check, particularly if the installation has never been tested.