We often hear customers exclaim "I didn't know I had so many spiderwebs in my house!" after a fire. What customers are seeing is not the handiwork of a spider - but a chemical reaction that happens after some fires.
The ionisation process of burning (particularly) synthetic materials like plastics, rubbers and polymers, creates charged smoke particles which can attract towards certain surfaces, and attract to each other creating chains, and ultimately web-like formations. They form under certain conditions called wet smoke, which involves low heat, or smoldering type environments. Smoke webs are typically sticky and pungent and will smudge easily and show up in particular areas where there is limited air flows, typically corners and edges of ceilings, in curtain folds, cabinets etc.
Spider webs will burn up very quickly in a real fire.