Freiren: How did Fern's group capture a Stille? Wild speculation and wrong answers okay
It's never shown or stated in the source, but that is the most logical answer. She was good at hiding her mana even before being trained by Frieren for most of her life up to this point.
I thought Frieren was still better at suppressing her mana.
Although, Frieren did state in an earlier episode that Fern was faster at getting off spells, which would definitely be an advantage here.
I think the running theme has been that fern has built in stealth hax, since frieren struggled to find her before she could even suppress mana and the demon Lord couldn't detect her even using a flying spell right outside the window he was next to.
Also in the episode it looked like she was walking back to the two others as they were waiting expectantly and then she asks what now.
If the birds use mana as a threat reading above all else she could probably just walk over to it and pick it up.