They're still not sure that she's the squirrel divine beast, but since the timing of the squirrel's disappearance and yureika medest's appearance are close to each other they felt it was suspicious
They might also have seen baby Yurika as a human/humanized once before, back when she was in the temple before her first meeting with Johan. Yurika clearly has gaps in her memory from that time as she doesn't seem to remember having met and healed the 2nd prince, before meeting Johan.
True , also it was hinted in the story(forgot which chapter) that medest family's children have been getting kidnappeddisappearing
There's definitely a link between the experiments done on "divine beasts" in the temple ,and medest children's disappearing, is what I think
How tf did those two put 2 and 2 together that Yurika is a divine beast. Did i miss something