Can mold in a bathroom pop up unexpectedly and rapidly multiply, like overnight?

I have a friend who has a roommate in 2 bedroom, 2 bathroom apartment.
My friend had reason to go into her roommate’s bathroom…and the sink and the tub are just covered in black mold! The roommate swears she has no idea where it came from.
Honestly, I haven’t cleaned my tub in months, and I don’t have mold.