1)An excuse for any kind of illegitamate situation. Covers anything from smoking to sex.
2)Actually going for a walk, possibly to see other people you've been looking for.
1)"The high school students 'went for a walk' after photography class. They were gone for more than an hour. Who knows what they were doing back there..."
2)Girl:"I'm going for a walk to see if I see that hot guy who lives back by the school."