Could be an advanced civilisation sucking all the stars energy into the back of their spaceship.

More likely just someone feeding all the mass of the star to a black hole for industrial purposes - eq. a Deep Well Industrial Zone: https://www.orionsarm.com/eg-article/464790d2497de

Orions Arm even has a story about how such process might look like: https://www.orionsarm.com/eg-article/46709da5de6be

You basically let material stream into a black hole, it forms an acreation disk which gets very hot and dense even before the material actually falls into the black hole. The temperatture and pressure is high enough to trigger nucleosynthetic fusion reactions that generate heavy elements from lighter stuff, like the abundant hydrogen and helium. And a lot of "process heat" that can be used as energy source for other purposes. :)

Perhaps a superadvanced civilization training an AI model on all remaining negative entropy in their solar system so they can more effectively create realistic propaganda for the upcoming election on their now rather chilly mars colony.

Or it could be the sun eating dragon :-)

Joking aside, it could be a Kardashev Type II (or higher) civilization. https://en.wikipedia.org/wiki/Kardashev_scale

They could have used a dense Dyson sphere to “suck” the energy of the star, but if that was the case we would be able to detect its infrared radiation. https://en.wikipedia.org/wiki/Dyson_sphere

Anyway, I prefer the giant star eating dragon alternative ;-)

And then where does it go?