AIDAN FRENCH woke late in his Randwick flat, made breakfast and waited for his flatmate, Sean Walsh, to emerge from his room. Walsh, a fellow Irishman, never surfaced that Sunday, May 10. "I didn't think anything of it because his door was closed," French says.

Only when another friend dropped by did the two men look in Walsh's room. "It was pretty normal," French says. "There were some clothes, his mobile was there, plus some books by the bed. But there was no Sean."

Three months later and there is still no Sean, his disappearance baffling family, friends and police, who say his bank account is untouched. There has not been so much as a false sighting, despite an extensive public awareness campaign led by Walsh's sisters.

"Usually you'll get at least one person saying they saw someone who looked like the missing person in Byron Bay or wherever,'' says Paul Pisanos, a detective inspector. ''In this case we have nothing … It's as if he has literally fallen off the map."

Trim, with short brown hair and an easy smile, Walsh, 26, was by all accounts engaging and intelligent. He was was not a gambler, did not take drugs and had no history of mental illness.

"He was handsome and popular, particularly with the ladies,'' said Siobhan NiFhaolain, of Canberra, Walsh's godmother. "He's the last person you'd expect to disappear. But we do know now that something happened to him in those last few days that troubled him greatly.''

A biomedical engineer by training, Walsh left his home town of Dungarvan, Ireland, in 2007 on a round-the-world ticket. A keen skier, he worked in ski resorts in New Zealand and Japan, then backpacked in South-East Asia. He arrived in Sydney in June last year, and promptly visited NiFhaolain in Canberra. "We talked about the family and his sisters and his skiing. He was perfectly laid back and happy."

Walsh returned to New Zealand but was back in Sydney in October, quickly settling into the life of an expat Irishman, playing Gaelic football and hurling, surfing on Sundays and drinking at Bondi Junction pubs like The Cock and Bull and Tea Gardens. By December he had a job with the carpet cleaner Absolute Services in Randwick.

"Sean was very dependable," says Craig Whelan, its owner. "He could be a little shy, but he was always very reliable." Whelan trusted Walsh, and often lent him a company van at weekends.

Walsh told Whelan he planned to return to Ireland briefly in late May. He had been planning the trip with his younger sister Aine as a surprise for their mother. On Wednesday, May 6, he went for a job interview with Advanced Surgical Design and Manufacture, a company in St Leonards that specialises in prosthetic implants and surgical tools. "Sean came back saying the interview had gone well," Whelan says.

But on Thursday Walsh came to work "acting a little odd". Later that day Whelan started fielding calls from clients asking why Walsh had not turned up. "And so I called Sean and asked him what was going on. And he said that he was going home to Ireland early, that something very bad had come up. I said to him, 'What, like a death in the family?' And he said, 'No, it's worse than that.'''

On Friday a still agitated Walsh told Whelan he would book his air ticket that day. "I said, 'OK, take the van for the weekend and I'll see you on Monday and we can sort out your pay and everything then.'"

On Sunday morning Whelan found the van in his driveway. "There had been no phone call from Sean to say he was going to do that. The van's door was shut but unlocked, and the key was in the ignition, with all his stuff in it: his surfboard, wetsuit, rash vest, some paperwork of his, a T-shirt.''

Police say there is no evidence of foul play, but can offer little more to Walsh's family.

"The destroying effect of this is unimaginable," Aine says.

"I am not sure how my family still get up in the morning or go to sleep at night.''