Help understanding what happened to my dog? (Not for the squeamish)

General history: I have an 11 year old Malamute mix, 36kg, who is convinced she's still 3 most of the time. She has mild age related arthritis in her hips and was spayed late at age 7 due to split-seasons. She has a benign, unchanging fatty lump in her left armpit that was assessed 3 years ago and deemed ok to leave alone by two vets. She also has believed to be congenital corneal dystrophy with ok vision, unchanged since adoption at age 4.

Incident: On Tuesday evening, she had some belly scratches from us, was absolutely acting normal and about to be fed, when I turned round to see her standing in a puddle of fluid. Given her age, I assumed maybe she had an accident, but she looked confused and was looking behind her. At that point, I noticed the fluid, pink tinged and straw coloured, was actually pouring from her right side. My husband came in when I shouted for him and observed a roughly 2 inch diameter hole in her side, which when I looked in, I could see deep into the subcutaneous cavity. Emergency vets were called, they irrigated the cavity and disinfected the wound, clipped her fur and gave us a cone, a bag of saline in case we needed to re-wash the injury, tons of sterile gauze and some vet-wrap that ended up not staying well enough in place, but at least stabilised her for the night until we could get her to a clinic for emergency surgery to close the wound.

After the surgery, she was left with a Penrose drain and a follow up appointment on Monday for a wound check and drain removal. Apparently the cavity went much deeper under the skin than we had seen, spanning a good portion of her torso. Looking at her now, she's much smaller on the side with the hole, leading us to believe there has been quite a bit of tissue loss.

What I don't understand is what happened? The fluid was not purulent, had no infectious smell, and she seemed to have no pain at all, and kept trying to run around while her side was gaping open! She hadn't had any changes in appetite or energy, no fever or behavioral changes at all, but how could such a large buildup be painless? Especially once it tore through her skin? And how did we not realize? Shes a very fluffy girl, but she loves her belly rubs so she always has hands on her regularly.

Any ideas would be welcome, as I can't wrap my head around this potentially being an abscess of that massive size with zero systemic signs or symptoms.