"it's not safe to assume anything"

Sometimes you may want large boulders for use in landscaping. Otherwise they should be the excavator's business to remove. Stone is clean fill, so it shouldn't be a problem. Trickier things we've come across are half-filled oil tanks, glass-filled landfills and ash from prior business operations. Removing and safely disposing of contaminated soil can be a very expensive venture. Excavators will likely write themselves out of responsibility for handling such discoveries.