The viability of self-replicating drones depends on the materials the alien technology is composed of.
Let's imagine self-replicating human drones. Our technology is based on circuits and computer chips that require metals like copper and gold to transmit electrical signals. For chassies we can use a wider range of materials like metal, plastics, glass or even wood. Each material influences the properties of a drone like wheight and robustness.
If the alien technology is based on the same materials, they can harvest (or demolish) existing infrastructure like railways, junkjards, factories or cities to replicate. If they choose an industrialized country as starting point, these activities will be noticed very soon and humans might destroy the drones before they can replicate the necessary numbers. If they choose an undeveloped or uninhabitated area, they might lack the raw material to replicate.
If the alien technology is based on completely different materials than human technology they might be able to hide their activities far longer. It would take us some time to notice aliens harvesting sand in the Sahara or carbon from the atmosphere.
It might also be impossible to replicate enough drones due to a scarcity of the necessary materials on Earth.