They're insect-like creatures; beetles perhaps. They build stuff out of available materials, progressing toward some unknown objective. If they're in a house, they might first take small bits of available metal like paper clips, nails and kitchen utensils and turn them into small tools and wire; then they'll build a small crucible, lathe and other machine tools. Within a few weeks they're building small electromagnets, motors, batteries and steam-powered and internal combustion engines. At the same time, they build little defense mechanisms to protect their work in progress (traps, firearms, barbed wire, electrified fences, etc.) If left alone they work their way up to purifying silicon and crystals, complex servomechanics, etc.— it's a Big Machine with an unknown function.
Could work. Why not write it up?
EDIT: Goddammit and you already did. That's the second time today ;[
EDIT #2: I like this. Entertaining.
Might as well delete the thread now, since it's up as SCP-831.