Instant of Eternal
Only people who are Symphonically Aware can perceive Celestials in their natural form. So, a Celestial never needs to worry about a crowd in a mall seeing them flit about in their natural form (unless the mall is filled with Soldiers.)
People who are Symphonically Aware will perceive things in Celestial form in the same manner as they perceive things in Corporeal form. As such, stealth checks still apply (using perception rather than dexterity).