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A solution can be

• Detecting presence or not --> background subtraction, since the place where an object will be is known
• The result can be cleaned using morpholocal operations and blob analysis, both in openCV.
• Pass the bounding box of the car to the ANPR interface

You could even optimize this and take only a ROI of the found blob, since you will know due to the fixed cam, where the license plate will be with some buffer.