With the R134a systems, they use a machine that evacuates or takes up the old FREON, or new FREON - but they also monitor the system using a scanner on the OBD port.
One of the things we used to do is just fill it up with FREON and be done with it, but todays newer cars have several critical components in the AC unit - so the system needs a specific amount - and no more.
Been finding that out as I ask questions with the various shops, I've visited
So for them to say, we checked it - seems kind of fishy - more like something else they had to do but can't tell you.
Remember that there is a lubricant in with the Freon they have to use a specific type too, so that amount as well as type can mean they may have found the pressures not right so the balanced it in using their machine to get it to work again.