Administrative assistant Shoba Devi, 27, got suspicious when her Filipino maid of a few months started chatting on the phone late into the night.

Miss Devi, who is single and lives with her father who is in his 60s, said: "I didn't know if she had a boyfriend and I was worried for our safety if she brought her boyfriend home."

In February, she hired private investigator James Loh, who runs International Investigators, to keep tabs on her maid during her days off.

He discovered that the maid - in her 20s and married - had a Bangladeshi boyfriend and, on her days off, they would meet to eat, shop and check into a budget hotel.

Miss Devi said: "I kind of expected her to have a boyfriend but I was shocked that she checked into a hotel with her boyfriend. What would happen if she got pregnant? So I sent her home."

She paid the private eye about $2,500 for three days of monitoring the maid, but felt it was money well spent.

She said: "It's for our safety as the maid lives with us."

Like her, some employers are forking out thousands of dollars to check on their maids. Private eyes charge about $1,000 for a day of surveillance and say it can cost between $1,000 and $4,000 to tail a maid.

Six private investigators interviewed said they all get regular assignments every month to track maids, and the number doubles during the June and December school holidays when families go on holiday and leave their maids home alone or with an elderly parent.

APAC Investigation & Consultancy's Raymond Lim said he used to get one such job every two to three months in the past. Now he attends to one or two a month and maid surveillance now accounts for up to 20 per cent of his business.

Mr David Ng, director of DP Quest Investigation Consultancy, said: "It is every employer's nightmare that some unknown man could intrude into their house. You don't know what these intruders could do, like turn violent or steal things."

While Singapore employers have long turned to sleuths to trail their maids, the investigators interviewed said demand has risen since it became compulsory last year to give maids a weekly day off.

Many employers worry about what their maids get up to on their days off.

The Manpower Ministry (MOM) receives an average of 200 complaints a month about errant maids. In a report last week, maid agents told The Straits Times that bringing boyfriends home, stealing and hitting the children are the top three complaints employers make about their maids.

Some employers also get help to check if their maids are involved in prostitution - something the private investigators say is not uncommon.

Mr Ng said he trailed a Filipino maid in her 20s and found that she would check into different budget hotels with different men on a Sunday. She spent her days as a sex worker, with three to four clients each day.

His client let go of the maid as soon as he reported what he found out.

Some maids have been found to bring their boyfriends home, just as employers fear.

Mr Lim was hired to monitor an Indonesian maid when the family she worked for went on a long holiday last December, leaving her with an elderly woman with dementia.

He found that she would sneak her foreign worker boyfriend into the house in the middle of the night and the man would leave before daybreak.

When the family returned and found out, they let go of the maid.