Maid had a good explanation

. . . for saying she was single when the truth is she was already married and had four children.

When she applied for a passport she declared herself single because could not produce her marriage certificate. Since she wanted so much to come to Singapore, she took the path of least resistance and declared herself single. So in order to be consistent, she declared herself single when she filled in her application for a position in Singapore. It was easy. No one bothered to check, no one cared. A woman may not be able to produce her marital certificate for a variety of reasons:

she has been married for such a long time she no longer remembers where her marriage certificate was;
sShe did not have a civil marriage so there is no record of her marriage in city hall;
her marriage certificate was consumed by white ants; or eaten up by mice or the cat or her cousin's pet rabbit;
her marriage certificate was probably destroyed when Mayon Volcano or Mount Fuji erupted.

So when Ma Lhudie came to work in Singapore, she had to be consistent and declare she was single in her application too.

Women who lie about their marital status can be found out in a variety of ways. Eg:

they absent-mindedly make reference to their children or their husband;
they carelessly leave lying around a family photo with spouse and children;
they are betrayed by their erstwhile best friend now bitter enemy
they receive unfavourable news about husband or children and are unable to keep their dark secret any longer.