I was an asbestos consultant in the 90's and I kinda agree with what you wrote. Like you said, it's probably fine but it COULD have been contaminated during a renovation at some point. Asbestos was used in just about everything under the sun until the 70's and was gradually phased out during the 80's. Raw asbestos is fibrous but if it's processed far enough it just looks like dust and you can't tell for sure without looking at it under a scope.
I oversaw a lot of (flat) roof jobs that ripped up asbestos/tar membranes and put down 2"-3" foam board covered with a new membrane. Most contractors I met will bid to toss whatever insulation was underneath because they couldn't guarantee it's condition or performance. So I also agree that makes it a likely source for lightly used foam board. Don't know what the red stain would be though. Any gruesome murders in the area lately?