Investigate
His feelers were able to give him valuable information. Information that he could use at a later date to track him down and avenge his men.
If there is a later date Garrus thought. Without his men providing support, his chances of survival against the mercs was slim.
The information was about the recent transactions Sidonis had done. Just before the attack, he had booked transport off Omega to an unspecified place. He also cleared out his private accounts. In conclusion, he sold Garrus out and ran.
Coward Garrus thought. The worst thing a turian could do was lie. Every turian was taught to own every decision they made, good or ill. If Garrus catches Sidonis and questions him directly about the murder, he'll likely confess.
But he'd need to find Sidonis first before he could catch him. Garrus informed his feelers to keep track of anything that could lead him to Sidonis. Even if the lead was slim, it was better than nothing.