Merrill is subdued herself, a clear sign that not all is well with the elf. She's come across one of the things Nathaniel has accidentally left behind, an arrowhead that perhaps came off one of his arrows, and it provides an excuse to ride up to him as they return to Skyhold.

It's offered with an outstretched hand, though Merrill keeps her eyes foreward.

"What happened in that building, while we were outside?"