Does mould growth definitely indicate a leak?
Does mould growth definitely indicate a leak?

No, not at all. Mold can grow in any suitable environment, which just means sufficient moisture and sufficient food.
Unfortunately, neither of these need be present in very large quantities.
It's relatively common to find mold growth on the ceiling in a poorly-ventilated bathroom, or even on other surfaces. Likewise, on walls in a humid basement.
These issues should be addressed with a two-pronged approach: improve the ventilation so that the humidity is kept low enough to prevent mold growth, and on painted surfaces use paints specifically formulated for humid environments and which have mold-retarding chemistry.
Of course, none of this means that the roof isn't still leaking. If you're at all concerned about that, you should go check it again to make sure moisture isn't getting through the roof. But certainly, the presence of mold doesn't prove that the roof is leaking.