Detectives from South Wales, who have spent almost three weeks looking for Richey James, - guitarist, lyricist and one of the driving forces behind the band - have contacted coastal authorities to see if any bodies have been washed up on the Bristol Channel shoreline.
Police said that many of those who committed suicide by jumping from the bridge, hundreds of feet above the water, are never found because the strong tides and powerful undertow often take their bodies out to sea.
But the officer leading the case, Detective Inspector Frank Stockham, yesterday said he was keeping an open mind on James's fate as the service area on the M4 was near a junction with M5 which would open up much of the country if he was picked up or had hitched a lift.
The locked car of the tormented 28-year-old star- who was last seen on 1 February after walking out of his Bayswater hotel in west London - was discovered on Friday afternoon after a new attendant noticed it had been parked there for several days.
Staff at the station did not recall seeing James - who has a history of self-mutilation and suffered a breakdown last summer - and so it is uncertain how long the car had been there before it was noticed.
It is known that after leaving the London Embassy hotel hours before he was due to fly to the US on a promotional trip, James - full name Richard James Edwards - went back to his flat in Cardiff Bay where he left some documents and his passport.
Yesterday, the management of the punk-revivalist-cum-rock band, which has had to postpone a 30-concert tour of the US due to start tomorrow, refused to comment on the latest development.