This is one of those specialist technical terms that lurk in the interstices of the language for a while, but then suddenly pop out, catching word researchers by surprise. Though it’s well known in archaeology and has been around at least since the early 1970s, it appears only rarely in dictionaries.
An ecofact is a find at an archaeological site which comes from something living, but which has not been modified by human activity. Examples are wheat seeds, sheep bones, or seashells at inland sites. Finds like these tell us something about the diet, way of life, or culture of the people who lived there.
Something that has been created by people is an artefact (an artifact if you’re American); ecofact was formed from it by blending it with ecology. The fact part of artefact comes from Latin factum, something made, so ecofact might mean something created from a living organism, exactly the opposite of the way archaeologists use it. However, it is equally possible to parse it as something made by a living organism, which would release archaeologists from the accusation that they’re bad at etymology!
Other terms of like kind include geofact, naturally fractured rock that looks as though it might have been manufactured by human action, but hasn’t, and ventifact, something that has been shaped by wind-blown sand.