Shepard, of course, was livid when she found out.
He hadn't meant for her to find out – spirits, he hadn't planned on this at all. Garrus had left the refugees hours before in order to clear his head, because you don't talk to the scores of turians fleeing Palaven without being reminded that your home, your friends, your family are being attacked by Reapers this very instant. The injuries, the death tolls – too much for one turian to handle, not all at once and through the cold statistics being broadcast every day.
So he'd taken a walk – clear his head and all that. The SSV Normandy SR2 was docked, and the crew was given shore leave; well, as much as a shore leave as you could get while your home planet was being destroyed. Garrus wasn't worried about wandering about the Citadel for a bit, not if it could stifle the maelstrom of "what ifs" and the never-ending sinking feeling in his stomach.
He'd ended up in one of those sketchy docking bays, shuttles and crates strewn everywhere, with clientele as unsavory as you'd like. Perfect place for smuggling a few guns and what have you. Not the safest of places to be, but he was a big turian now and could handle himself.
At least, that's what he thought when he was behind a stack of supplies and heard the armored men mention the Normandy – a quick glance around the crates and a Cerberus logo was all it took for him to charge in, guns blazing. It was fine up into the point there were one too many for him to handle alone – well, maybe it was five or six too many, all a bit more well-trained than he expected. That's how he ended up in the infirmary, with Chakwas fussing over the gash on his wrist Shepard shouting up a storm.
Shepard wasn't angry about him taking out some Cerberus lackeys – it's the fact that he didn't send for help when they started to overwhelm him. They needed the crew in one piece, she reminded him, as Chakwas applied something that smelled like absolute shit to one of his various burns.
Garrus took the shouting and swearing in stride, occasionally biting back a quip or two. He prefers Shepard like this – not the angry part of course, but a Shepard that isn't focused about the big things, like Reapers and idiotic bureaucrats and insane pro-human groups, was a better Shepard. Taking out the lackeys alone meant one less set of soldiers for Shepard to worry about, and one less worry for her when the whole galaxy was depending on her. Pulling stupid shit like this means Shepard is mad at him, and not thinking about the thousands of things that are far worse.
So if she wanted to swear and shout at him for taking on Cerberus by himself, that was fine by him. He could handle it, especially if it took a little weight off her shoulders. Because in the end, Garrus know that all his worries - about the survival of his home, his family, and the refugees he has to face – are nothing compared to that of a woman to whom the entire galaxy had turned for help.