After a bit of quick testing:
The campfire causes a 5 when you turn it on.
Clean up one shopping trolley 5.15
Scrap Iron piles, 3 each.
"Wood trash" piles, 1.2.
Windows, weak iron doors ~3 each.
Flimsy iron sheets (one-shot), ~3 each.
Breaking down a zed closet door 1.3 ..
One iron ore node 5.
So, show up at a house, start boiling water, clean up a couple trash items from the environment and you're at 25 easily.
If you leave a campfire running permanently, for RP or lighting reasons or w/e:
1 campfire + 1 torch, causes peaks above 20. Alone.
My third intentional crossing of 25 spawned a pair of screamers.
None of the three attempts were things I couldn't do on day 1. One small mining session, one with a couple campfires, one with a "clear a POI and start cleaning it". I was focusing on things that would cause heat, for sure, but not exactly anything out of the ordinary.
This seems to be the design as it was released, I don't really see the need for video evidence..?