The holder of currency takes free of a security interest over the currency unless they have actual or constructive knowledge of the existence of a security interest over the currency1.
Currency neatly bears that the object of the extinguishment rules is largely to keep the economy ticking over – currency simply must be freely exchangeable.
The extinguishment rules under the PPSA are completely silent about title to currency. Section 48 is simply expressed to apply to security interests over currency. The question that may occasionally arise is whether the extinguishment rule in section 48 washes-clean title to stolen or otherwise non-owned currency. Rather, does it merely extinguish security interests in favour of buyers or lessees in otherwise valid transfer transactions. See the discussion above at paragraphs 21.1.12 to 21.1.40
Realistically, currency can be fungible and hard to trace, and extremely easily dissipated. Security interests would be very easily lost over currency if it is spent and the secured party cannot identify and trace the proceeds.