That’s actually a really good question! Thank you for sending me this.
From Tara’s point of view, I can see how she could think that Maggie could think such a thing because Tara automatically feels guilty over everything, even over things she didn’t do. (Example; Hershel’s death, she categorized it as worse than anything that had happened to her family.) I believe she feels guilty over the fact that she got to spend time with Glenn that Maggie will never get to make up for. Yes, they reunited- yes, they’re stronger than ever, but Tara still spent time with Glenn that can never be replaced. Every day in this world is precious. And in a way, Tara feels responsible for taking that time away. She was part of the attack of the prison, whether she fired a shot or not, she was there. If it wasn’t for that attack, Glenn and Maggie would have never been separated. And since she’s the only survivor of the attacking side (That we’re aware of) she’s going to carry all of that guilt.
Do I think Maggie thought Glenn and Tara did something unfaithful? No. Absolutely not. Those two have been through everything together stringing from a pregnancy scare, to a possible death scare when Glenn got sick. They trust each other whole-heartedly. And while Tara is a stranger and Maggie may not trust her right away, she trusts Glenn. She hugs Tara in that tunnel because Glenn said Tara was the reason he got back to her. I don’t think that thought ever crossed her mind. When Maggie looks at Tara over Glenn’s shoulder I didn’t interpret it as a ‘what did you two do while I was gone?’ look, I saw it as a ‘You brought my family back together and I’ll never forget it’. When Tara finally tells Maggie that she was there at the prison, Maggie never strikes her, or gets angry, or demands that she be exiled from the group. She simply hugs her and says ‘You’re with us now’. When Maggie looks at Tara she knows what she’s lost. She knows Tara lost everything and yet she still devoted her life to save her husband’s.