Drinking too much
beer in Munich ;)
Join Date: Feb 2012
Location: I would rather not say
From what little you can see with the street lights shining through it I would say it's steam from the way it disperses so quickly.
One of the oxidation actions of the Cat is HC + O2 → CO2 +H2O (carbon dioxide + water)
The amount of visible steam you see (instead of less visible water vapour) depends on the dew point of the air and the humidity.
In West Lothian right now the humidity is about 94%, every car is steaming, and the dewpoint temperature is well above ambient. This means that the water vapourt which yoiu can't normally see, because it is invisible vapour, becomes very visible.
I'm only saying this because it seems to disperse quickly, but there isn't quite enough footage to be sure.
If yiou aren't losing coolant and the oil is in good condition I don't think there is much to worry about.