Many cases are solved by coincidence, and many conspiracies are exposed due to a minor accident. Gao Guang just happened to arrive at the right place at the right time and, furthermore, he was careful enough to notice some clues.
Earning a PMC's salary while doing detective work was no problem, but asking Gao Guang to take out Glaev was out of the question because protecting his employer was legal, and he could even legally kill an intruder threatening his employer's safety. However, he had no law enforcement powers, so while passive defense was okay, taking the offensive was not.
So all he needed to do was call in any clues he found, which wouldn't even interfere with Gao Guang's meals, but just half an hour after he had barely finished his lunch, Danny's call came through.