On a trip a few weeks ago traveling out of Charlotte (CLT), I took this picture of a vehicle at the airport. Our plane was fueled as we were about to board but due to various factors, boarding didn’t occur until almost five hours later. As we were finally about to board, this vehicle arrived and connected a tube to a port in the vicinity of the fuel input port. My question is this: what was the vehicle doing and what circumstances necessitate the usage of it?
Edit: Although not immediately obvious without zooming in, the yellow pieces visible are the ends of a spool around which the hose is wound. It is not one single large tank as it appears from a distance.