yes, and depending on what had deposited on the board surface before hand it can be on the same roder as what you saw there,
There are different types of fluxes, but even if you use a non conductive one, the metals and oils that deposit out of the air over time, once it has a medium to move around in like molten flux tend to concentrate in, can cause exactly what you are seeing, and is part of why cleaning off the flux is a good habit to follow,