Mold on the surface
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Why the surface
Mold needs available water and air. The surface of the syrup has both in a way the body of it does not, especially if condensation collected in the headspace as the bottle cooled. So a syrup marginally too thin fails at the top first, which is also where the buyer sees it.
The first thing to check
The density reading for that batch, with its temperature and which instrument mark was used. If the reading is missing, that is the finding: you cannot tell an under-density batch from a packing problem without it, and the two have completely different fixes.
The second thing
The fill temperature at the bottle the complaint came from, if you can place it in the run. A batch at the right density that molds in the last few bottles of a run is a cooling syrup being packed, not a thin one.
The third
The container. A bottle that was not clean and dry, or a closure that did not seat, puts both contamination and air against the surface. This is the least common of the three and the one people check first because it is the easiest to inspect.
What not to do
Do not skim it and re-sell the bottle, and do not treat one complaint as one bad bottle. If the cause was density, every bottle in that batch has the same problem and most of them have simply not shown it yet. The batch code is what lets you answer that question instead of guessing at it.