When Kestrel stumbled upon Ren, he was hiding in a tiny, cluttered cabin. Despite being surrounded by mess, he looked somewhat okay, not as worn out as she had pictured. He was slouched in a gloomy corner, his hair wet and dripping, and his entire body covered in a strange chemical liquid.
Kestrel identified the liquid — it was a mimicry substance.
The first time she met Ren in the Polluted Zone, he had used a little of this mimicry substance, tracing a circle on the ground near her with his finger. This circle blocked all the human smells coming from her, making sure the mutants walking around outside couldn't find her.
Now, either afraid of being found by another sharp-smelling alphas or trying to cover a strong odor, Ren had soaked himself thoroughly with the precious substance.