No, those are registration numbers and are easy to find because as you said, they're visible on the outside of the aircraft.
The manufacturer's serial/construction number is different and is unrelated to the military serial. It's assigned by the manufacturer and is almost always hidden away on the inside of the aircraft.
Either someone's leaking these numbers to the public or they're intentionally made known to the public under freedom of information. If it's the latter, then I'd like to know what the source is.