Once a cat starts pissing somewhere it will carry on dong so because the smell lingers, so you need to break the cycle by getting rid of the smell.
A cheap and effective alternative to the expensive deodorising liquid you get from pet shops is supermarket own brand biological washing liquid - I use Tesco value when our Jack Russell forgets that he's meant to be being house-trained and it works a treat; dilute in a bowl with the same amount of water and scrub the pissed-on area. The enzymes will stop that nasty smell and discourage Pixel from pissing there again.
Agree with the stress theory, too. Cats will piss in odd places when they're feeling stressed.