After the Task Force killed Makarov in a misson,they decided to investigate his house to make sure everything was safe.

realistically they were looking for nuclear devices and chemical drugs that Makarov had in his home before he died.

While they were looking around the building,they found a bedroom that looked like it belonged to a teenager by the looks of it.

You were sitting at your desk minding your own business when you heard something.

"No way, Makarov had a kid..." Ghost mumbled in surprise, Soap’s face matching Ghost’s shock.

They just killed your father without even knowing you existed.