When he saw that the door to his flatmate's bedroom was unlocked, he crept inside and molested the sleeping woman.
Malaysian cook Ong Yong Chern, 23, was jailed for five months on Tuesday after pleading guilty to one count of outrage of modesty.
A second molest charge involving the same woman was taken into consideration during sentencing.
The 28-year-old victim, an administration officer, and her mother shared a flat with him as their home was undergoing upgrading at that time.
Deputy Public Prosecutor Krystle Chiang said that the unit belongs to the victim's uncle and that Ong was a tenant.
The two women shared a bedroom and they always locked the door once they were inside.
But the mother left the door unlocked when she went to the bathroom at about 6.40am on May 1 last year. Ong, who arrived home around 6am that day, sneaked into the room and touched the woman's left breast and buttocks.
She woke up and shouted when she saw him sitting on the bed beside her and he ran back to his room.
She informed the police at around 1.20pm that day and Ong was arrested.
In mitigation, his lawyer, Mr Josephus Tan, said that Ong had consumed a "considerable amount" of alcohol before the incident and could not even recall how he had managed to get home.
Mr Tan added that his client was remorseful for hurting his victim's dignity.
For molest, Ong could have been jailed for up to two years, fined and/or caned.
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