"left over"
Nah,
Ozone on Mars transforms freely back and forth to O2, and reacts with other atmospheric gases on a timescale of days to months, when irradiated by strong UV. (recall what happens to the Ozone hole on earth when sunlight hits it)
The Ozone on Mars now is in equilibrium with Mars now.
There is an annual (martian year) cycle of O3 and O2, linked to atmospheric pressure and CO2. If Ozone could be "left over", it would stay at the same partial pressure year round. Instead, it shows a similar mixing ratio, year round, proving that it is in dynamic equilibrium with the modern atmosphere.
Barry, if this is the best you can do, then I'm glad you're affilliated to Cardiff, and not any of my institutions!