Some people are easier than others to do this with; i.e. if a person is capable of talking for half an hour wanting a reply to make sure you didn't go on holiday, they are often easy targets for a stealth piss.
*** person 1 walks off to go to the toilet without informing person 2 ***
Person 2: "I'm fantastic!"
Person 2: "I got paid earlier so I bought some really awesome shoes in town"
Person 2: "And then I bought like 5 new CDs"
Person 2: "Which I'm listening to at the moment"
Person 2: "Oh and I saw your friend from school"
*** person 1 returns from urinating ***
Person 2: "He said you should come out sometime"
Person 1: "Yeah I think I should sometime!"
(person 1 has performed a stealth piss, unbeknownst to person 2)