SOULS_LEADER is right, it's in manga 5 (you know, the one where Sousuke is exclusively holding Tessa on the cover :evil: :evil: :evil: )
Anyway, if you scroll down there's the description in the manga... don't read if you don't want to know (don't say I didn't warn you)
Mission 31: Sweet Memories
Tessa decides that during her extra time before the crew rendevous with de Danaan she wants to do something for herself and takes a train ride all on her own. During the train ride she falls asleep and is awakened by the conductor at the last stop in Futatzutani. Tessa asks for the next train and there are none leaving that day, she tries to call for help but her cell phone isn't getting a signal. A stranger picks her up (honestly Tessa, riding with strangers *tsk*tsk*

) and she goes out to their farmhouse. The old couple there reminds Tessa of Sousuke and Chidori (see that... married, older couple reminding her of Sousuke and Chidori -

IT'S A SIGN

) and they treat her well as a guest.
The following day Tessa wakes up and joins the lady out in the field doing old fashioned labor, when they pass through a sunflower field.
"Wow! Look at all the sunflowers!"
"Aren't they pretty? The just all of a sudden opened up." The lady explains. "Seein' these flowers bloom like this... kinda cheers me up, you know?"
"Yeah." Tessa agrees and the next frame is of her waving with a sunflower in her hair.
So they do their labor out in the field, Tessa develops a new appreciation and when they get back to the house the gentleman is having a hard time with his truck. Tessa, being good with machinery, fixes it as a thank you. She then confesses that although she choose her job, she kind of doesn't want to go back to the responsibility. She thinks of the lives that depend on her, although she doesn't have any regrets, she wants to stay with the couple. The lady hugs her saying, "I don't know what your life's usually like, but you're a nice girl. And people who can be nice to others are also strong. You just stay the way you are... and no matter what happens, you'll do fine."
Tessa leans against her crying, thinking
Surrounded by plants and wildlife...
living close to the land...
If I hadn't been born a Whispered...
I think THIS is the kind of life I would've had.
Tessa thanks the couple for her time with them waving goodbye.
"Thank you! I won't forget you!"
She begins to feel tired again, reminiscing when she sees the familiar train tunnel and is suddenly back in her uniform having a conversation. She has asked Mardukas to look up information on the little town.
"Did you find the town like I'd asked? It was a wonderful place, and I want to send them a thank you letter."
"Yes ma'am, but... that town, Futatsutani... isn't even on the map anymore. It was flooded five years ago, during construction of a local dam."
"Oh..." Tessa's surprise fades to disappointment.
"Captain, that town is..."
"Mardukas. No. I guess I was mistaken." Tessa reasons before thinking.
It felt so familiar, so comfortable. I've never been anywhere like that before, so I SHOULDN'T have those kinds of feelings inside of me... But it DID feel that way... [visions of Tessa in the sunflower field again]
those feelings... are a part of me. And now... [outstretched hand]
And... I feel I can move forward again. Not as a sixteen year old girl, but as the CAPTAIN of the Danaan. [Stern look in eyes]
"Very well." She speaks to Mardukas. "Let me have your report."
"Ma'am!"