It's a context thing. If you're an ambitious person (and we know Claire is), and you're doing something nice for someone higher in the hierarchy, it's very easy for people to suspect some sort of ulterior motive. Especially if it seems otherwise out of character.
Her motive is actually kind of interesting to me. Her stated reason, not having to go to the coffee shop, seems kind of weird. But it's possible- I get the impression she's very focused, type-A and where the others see an opportunity to take a break she might see a wasted opportunity to accomplish something. She might be trying to ingratiate herself (kiss ass) with Marten, as Gabby was saying. And, admittedly kind of a long shot, she might actually be hitting on him in a sort of awkward, nerdy way. But that seems a little too Marigold for a plot arc. Oh, she could just want to thank him for the training (booooring). Lastly, she's just unwilling to go to coffee of doom for reasons unknown.
I reject her simply being really nice by way of her not getting anyone else anything.