One mouse isn't sleeping with the others?

We're away all day so I don't know about fighting (there's no blood or scratches or anything obvious), but one of the mice we got yesterday seems to be sleeping away from her two sisters.

She had a minor dispute over a pumpkin seed yesterday, but it was tiny- one mouse was eating the seed, and she came up and tried to take it. The other mouse pulled back and she left her alone.

I think this morning I saw all three of them in the house together, but tonight we had to search all over before finding her under the cardboard box they came home in. (They like to climb it, so we left it in.)

Yesterday she was the most active, climbing all over the place, and today she's hiding, which also might be a concern. (Or she could just be the most tired from exploring yesterday.)

They're in a 20 long gallon, so there's plenty of room for the three of them.

Is this something to worry about?