I noticed tree-like patterns on inner surfaces of home appliances plastic bodies. For example, on the lid of air conditioner indoor unit, inside of humidifier, at the bottom of washing machine powder drawer. The shape of the patterns reminds of Lichtenberg figures, but there's no source of high voltage inside those machines and no obvious reason of forming a high electric charge on the surface (and I've never been hit by a spark from them).
The following pictures demonstrate some of the patterns I found. The "branches" are near 1 mm wide. Contrast was increased a little.
The patterns can be easily washed away with soap and water, they are probably formed of dust. Air indoors is moderately humid in summer and very dry in winter, if it's important.
What is the reason of those patterns and what do they consist of?
EDIT.
Similar question Reason for strange “canyons” of dust on the cooling platform pointed by John Rennie shows the same dust patterns. The answer marked as accepted suggests that dust attaches to irregularities on plastic surface around marks from injection molding process. In my pictures the surface is free from such marks, but it can have salt deposit from water drops. The second answer on that question is very elaborate. Diffusion-limited aggregation of charged dust particles on differently charged surface is an interesting suggestion, that is the most probable in my opinion.
Similar question Dust patterns inside electronic product - what causes this? shows the same dust patterns. No answer is accepted. The highest rated answer describes deposition of statically charged dust particles onto plastic surface (like the previous one).
Question with the same pictures on Biology StackExchange. Comments state that dry plastic surface is unlikely to support life, but if it's a living thing, it's a fungal colony. Answer suggests that pattern can be a crystallization structure when liquids dry up. It can be plausible in my case (all mentioned machines deal with water that is splattered on plastic surfaces and dries up from time to time), but is not an explanation with electronics mentioned in other questions.
