Residue in V blender after cleaning

Our V blender was unused for a about 8 months, fully cleaned after use. There was some residue noted on inspection recently. Swab sample conducted and tests found no residual raw materials. What could be potential reasoning for the residue?

Please carry out microbiological testing of the residue/swab samples. It could be some fungal /microbial growth/contamination.
If not then investigate further for detection of any residue of the cleaning agent used previously or any other raw material/API used in previous batches.

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