Clad in black jackets with tags that say "airline baggage checks", they are stationed outside the glass doors which lead into the departure lounge. Their job is to keep a lookout for air travellers with a weight issue - carry-on bags that are too heavy.
Passengers who bust the maximum weight allowed - typically 7kg or 10kg - are sent back to the luggage counters to check in their bags. In almost all cases, they have to pay for the extra heft.
At least four carriers - Biman Bangladesh Airlines, along with low-cost airlines Jetstar Asia, Tigerair and Cebu Pacific - have hired Certis Cisco staff to make sure hand-carry rules at Changi Airport are not flouted.
Certis Cisco, which started offering the service in September 2011, now has close to 30 bag checkers. Biman was the latest to get on board last November.
All carriers have rules on the maximum weight and size allowed for hand-carry bags but full-service airlines tend not to enforce these strictly, industry experts said.
Budget airlines however rely heavily on ancillary earnings - money made from selling items and services aside from flight tickets, they said.
These also include charging economy passengers for check-in baggage, which full-service airlines provide as part of the fare.
By implementing the luggage checks, these carriers minimise revenue leakage of ancillary earnings, which typically account for more than $1 out of every $5 collected.
When contacted, both Jetstar and Tigerair said the weight checks are for safety reasons. "We operate a narrow-body aircraft and have limited stowage space in the overhead bins," said a Tigerair spokesman.
There are also safety limits on how much weight that each bin can hold, plus enforcing the cabin bag policy also ensure a smoother boarding process, she added.
The revenue collected from excess baggage payments is "negligible", she said, without disclosing figures.
AirAsia is the only major budget carrier at Changi Airport that has decided not to hire Certis Cisco's bag checkers. Instead, when its travellers check in, their hand-carry bags are weighed and tagged at the counters.
A spokesman for Certis Cisco who explained the work done by the bag checkers said there have been instances where passengers add extra items into their bags after checking in.
This is when its baggage checkers step in.
When The Sunday Times visited Changi Airport recently, a few travellers were stopped for carrying overweight bags.
In one case, all the baggage checker did was lift a traveller's haversack. She did not even need a weighing machine to confirm it was more than 10kg.
She said: "It's experience. One lift and I know it's easily about 15kg."
The traveller who was caught did not argue but was clearly not happy as he walked away, muttering under his breath.