There are several such gangs operating in Delhi and the National Capital Region (NCR) and they have struck big time this year, police say.
From January to Sep 15, the gangs have committed several big robberies, including an over Rs.40 lakh heist from a cash van in Karol Bagh, robbing a Chandni Chowk businessman of Rs. 60 lakh cash and stealing jewellery worth Rs.50 lakh from a Safdarjung businessman in Uttar Pradesh's Muradnagar.
Chandni Chowk businessman Mridul Jain, recalling the incident, said while he was driving down from Meerut in Uttar Pradesh he was accosted by two boys between 14 and 15 years of age who knocked on the car window and told him that oil was leaking from his car.
"When I alighted from the car to check the oil leak, my bag containing Rs 60 lakh cash was stolen from the rear seat of the car. The two boys were also missing," Jain said.
"The two teens were nabbed in May by the south Delhi police. They confessed to their role in several such cases," Deputy Commissioner of Police (South) B.S. Jaiswal told IANS.
In early September, Delhi Police arrested three men who belonged to separate gangs - identified as Ramesh, Suraj and K. Dinesh - from different locations in Delhi.
"The common factor among the accused was that they belong to Trichi in Tamil Nadu," Deputy Commissioner of Police (Southeast) P. Karunakaran told IANS.
The gangs got the "thak-thak" nickname as they knock on the windowpanes of cars and then decamp with the goods even before the owner has realised that he or she has been robbed.
Kulwant Singh, Additional Commissioner of Police, Special Staff (South), said such gangs have been operating for the past eight years, adding it is difficult to give them an exact number - but it could run into thousands.
"They are not organised gangs," he noted.
He said they mostly belong to Tamil Nadu and some other criminals of southwest and west Delhi have adopted the same modus operandi.
In Delhi, the gangs usually operate in the Inderpuri, Madangir, Nehru Place and Ambedkar Nagar areas.
The officer said that the gangs use juveniles to commit the crimes so that they would be freed soon after their arrest.
In the past year-and-a-half, more than 100 people of this gang have been arrested and Rs.3-4 crore cash recovered from their possession, the officer said.
Elaborating on the thieving method, Karunakaran said the members divert the attention of car drivers with several tricks.
"They deflate the tyre of a car standing at the red light. Then one of their members knocks on the car window to inform the victim that the car's got a flat tyre. In the meantime, another of their associates standing on the other side of the car steals valuables from the car," said Karunakaran.
Another method is to throw several notes of Rs.10 and Rs.20 denomination on the road and tell the car driver, "see your cash is on the road, pick it up".
"While the driver is busy in picking up the notes, one of the members steals valuables like laptops and other items from the car," he said.
They also divert the attention of drivers by telling them that fuel is leaking from the car, the officer said.
Sometimes, they also break the windowpanes of parked vehicles and rob valuables.
"Their modus operandi is unique because the anti-theft devices that owners install in their vehicles fail to track the car break-in," said the officer.
Gang members conduct recces to spot cars carrying huge amounts of cash, valuables or laptops.
"They pass on the stolen goods to other members of their gang waiting nearby to avoid detection," Karunakaran said.