If you're sure it's not another live circuit in the box, the next most probable cause in a house built in the '70s is a neutral-to-ground short or swap. I personally think it's unlikely to be a hot-to-ground leak, because you'd be blowing fuses left and right, including when you get your electric bill.
In houses originally built without three-prong grounding, "poor-man" retrofits were often done by hooking the ground to the neutral. This isn't a code-compliant path to ground as it doesn't provide shock-safety, but since the neutral of a circuit should always remain closed, it's at least an always-available path that's hopefully easier than through you, lessening the severity of a shock, and if a full hot-to-ground short should occur the fuse/breaker will still cut out.
Additionally, someone who didn't know what they were doing when they wired a plug in your home could have swapped the neutral wire for the ground wire. This is unfortunately rather common, especially in homes that have been retrofit to add grounding (the ground wire can't be bare and thus easily identifiable if it's not inside the Romex bundle, so someone could mistakenly hook up the green-jacketed wire as neutral, or could conceivably be faced with a box that has two indistinguishable white wires). when something is plugged in to that outlet and turned on, any other ground wire that is more directly connected to this outlet than to ground, or which is along a series path to ground, will be energized.
Both of these will pass a home inspector's basic tests with a three-prong tester; the average tester cannot tell the difference between neutral and ground, as they are both connected to the same bus strip on the service panel (which, confusingly enough, has further connections to both the common wire going back to the power company, and to your home's plumbing aka earth). A multimeter can help detect N-G "shorts" (there will be some resistance between neutral and ground normally, but virtually none if the neutral was shorted to ground within that box), but really the only way to be sure your home's wiring is safe is to open up every outlet and switch in your home and ensure they're wired correctly.