The victim did not see the person filming her in the shower with a mobile phone.
Even when police confronted the suspect - muay thai trainer Chaisim Santi, 38 - they found no videos on the two phones he owned.
But the victim, 39, recalled that the phones were white. And both of Chaisim's phones - an iPhone 4 and an Aurora - were white too.
After initially denying he had secretly filmed the woman, he admitted to his crime.
Yesterday, Chaisim was jailed for 10 weeks after he pleaded guilty to two counts of insulting the woman's modesty near her rented room. A third charge for a similar offence involving the same victim was taken into consideration during sentencing.
Deputy Public Prosecutor Star Chen said: "Both mobile phones matched the description of the phone that the victim saw in her toilet. The accused denied having placed any mobile phones in the vicinity of the victim's toilet.
"He further denied having had any intention to record the victim bathing. Police officers conducted a check on the accused's mobile phones, but did not find any videos or photographs of the victim."
But in a later interview, Chaisim admitted that he had deleted the incriminating videos.
He had heard the woman in her bathroom shower at around 12.30am on Oct 13 last year and decided to film her. The Thai national retrieved his Aurora mobile phone, positioned himself near her bathroom window and used the device to record a 20-second video.
He was recording another video of her in the shower at around 2.45am three days later when she spotted a white mobile phone sticking out from the lower corner of the window and screamed for help.
Her room-mate alerted the police at around 4.10am and when officers arrived, they noticed that Chaisim, who was still in the vicinity, owned two white mobile phones.
In his mitigation, counsel Kalidass Murugaiyan urged the court to jail his client for between eight and 10 weeks, adding that only one victim was involved. For each count of insulting the woman's modesty, Chaisim could have been jailed for up to a year and fined.