A group of SMRT bus drivers from China, who refused to report for duty earlier this week over a wage dispute, were seen entering the Police Cantonment Complex late Wednesday morning and have remained there ever since.
The complex houses the Criminal Investigations Department (CID), among other police units.
The Straits Times understands that the drivers are assisting the police with its investigations in connection with a report filed by SMRT.
No arrests have been made.
The Government on Tuesday said it plans to take action against the bus drivers from China who refused to turn up for work because they were unhappy with their wages.
It also described what they did as an "illegal strike", and said the bus drivers would be dealt with if found guilty.
In Singapore, strikes are legal if they follow rules stipulated in the Trade Unions Act and Trade Disputes Act.
Workers in essential services such as public transport and postal services are, however, a special breed governed by the tougher provisions of the Criminal Law (Temporary Provisions) Act.
These workers can go on strike, but they must follow special rules like giving their employers 14 days of notice.
The criminal law that spells out the rules for workers doing essential work to strike legally has been in place since 1955, with more categories of workers added over the years. Public bus and air transport workers made the list in 1967.
Those who flout the law, including the ringleaders, can be arrested, tried in open court and jailed for up to 12 months.
Since their employer SMRT provides an essential service and the China-born drivers did not give 14 days' notice, the strike was illegal.