(I started watching RWBY around the time 3x08 aired but I didn’t become active in the community until after volume 3 so I apologize if this theory was already made when it was more relevant)
So we all know that prior to being released in 3x10, Kevin (the dragon) was inside a huge mountain. He was dormant, or frozen inside the mountain, until something or someone woke him up.
But what froze him there in the first place? We hear from Qrow in 3x12 that Ruby “froze” Kevin and we can see in Salem’s monologue later as well as the Volume 4 intro that Kevin seems to be completely encased in stone.
Someone as inexperienced as Ruby, who used her SEW powers for the first time that night, could only turn Kevin into a statue. But what if a SEW much more powerful than Ruby could encase a Grimm inside a mountain completely? I think a silver-eyed warrior may be the one who froze Kevin in that mountain in the first place.
Addendum: Team STRQ was “pretty well-known” back in the day. Ruby mentions while talking to Summer’s grave that she stopped some bad guys and says “like mother, like daughter”, implying that Summer and her team did some cool bad guy thwarting when they were younger. Qrow knows about Summer’s SEW powers in particular (“You’re special the same way your mom was”) and while he could have just found out from Ozpin I feel like he’s probably seen Summer’s eyes in action.
So my theory is that Summer was actually the one to freeze Kevin in the mountain.
(Another side note - Glynda knew it was there. Was she just told about it by Ozpin, or did she witness the battle herself? I put her around STRQ’s age though she could be anywhere as young as early 30s, and she had to have gone to Beacon - or another school - but Beacon’s the most likely - to become a Huntress)