Arcane Sense currently reads like this:
ARCANE SENSE FEAT 1
Prerequisites trained in Arcana
Your study of magic allows you to sense its
presence around you instinctively. You can cast level 1 detect
magic as an innate spell at will. If you are a master of Arcana, it
becomes level 3, and if you are legendary in Arcana, it becomes
level 4.
It seems as though it's missing level 2 detect magic at expert level. I can't see why this would be intentional.
Actually, I just read the spell better. Turns out I'm an idiot.
Disregard.