The Forrest and uninhabited area in the Pacific Northwest is HUGE, plenty of room for 1500+ Sasquatch to live undetected.

I can't answer the infrared / imagining question but you would have to automate the detection, because manual review of the images would be time prohibitive. i.e. Is that a large Deer, small Elk or a female Sasquatch?

But I'd bet it wouldn't be too hard to filter for all "human" like contacts.

Still, you would need a wide angle, satellite or high altitude "snapshot", so you could process a "moment in time" for a large area.

Flying a drone or an airplane in a search pattern doesn't work for a bunch of reasons, one being time. The area to cover is just to HUGE.