I found out it was unintended and a fun fact.
In 2000, Vic Gabarini published in interview with the band, on Revolver Magazine. Here is an extract :
Revolver: On Roxanne, the band's first big hit, there's a laugh followed by a weird piano chord in the beginning. What the hell was that all about?
Copeland: Tell them what really happened, Sting. Tell them about that brilliant musical moment of yours.
Sting: I was just about to sing the first line of this celebrated song when I noticed there was an upright piano next to the microphone. I was feeling tired - I'd been up all night for some reason - so I just sort of sat down. I though the piano lid was closed, but it was open, so I wound up playing this incredible chord with my a$$. It was this atonal sort of cluster that went really nicely against the chords we were playing. We thought it was funny, so we left it in.
Summers: And real musicians can tell he's playing more from the left buttock than the right.
Sources : The Police website, Sting website