NSRI CEO Dr Cleeve Robertson said the buoy had been placed there in case of further emergencies.
“We are appealing to the public to please be vigilant and watch our buoys. If you find them, please give us a call and we will try and replace those buoys as soon as possible,” he added.
While the NSRI hopes the missing buoy will turn up, it has replaced the one at Rocklands Beach in the meantime.
The four teenagers from Burundi and the Congo had found themselves in difficulty at the beach on Sunday.
They were “friends who were caught in rip currents after entering the water to swim when a wave washed over them”, said NSRI spokesperson Craig Lambinon at the time.
The first body was recovered on Sunday, while the second and third were recovered late on Monday.
The last body, that of 18-year-old Uwamungu John Damour, washed up on Tuesday morning and was secured and retrieved during safer sea conditions on Wednesday morning.
There have been 50 successful assisted bystander rescues to date on the South African coastline since the pink buoy project started in co-operation with local municipalities, donors and sponsors in late 2017.
Depending on the area, between 10 and 25% of the buoys have been stolen.
They are hung on strategically placed signs in the hope it reminds people to take care when entering the water and not swim if lifeguards are not on duty.
The sign explains how to use the buoy as an emergency flotation device. It also provides a contact number for the closest emergency service and sea rescue station.
Those who would like to get involved with the project can sponsor a buoy.