I think that getting the grave found at the other end is a less serious problem than building it to last. If they have nanotech, they can explore the entire surface of the earth in great detail, including doing an ultrasound scan of the entire crust. Also the thing would have a magnetic signature, being metallic. And if you were really concerned, you could build in a powerful permanent magnet, which would make it even more detectable. You could even use temperature differentials to power a weak radio transmitter, but honestly that’s probably making it too easy to find. Better to have a whole host of slight anomalies.
I think that getting the grave found at the other end is a less serious problem than building it to last. If they have nanotech, they can explore the entire surface of the earth in great detail, including doing an ultrasound scan of the entire crust. Also the thing would have a magnetic signature, being metallic. And if you were really concerned, you could build in a powerful permanent magnet, which would make it even more detectable. You could even use temperature differentials to power a weak radio transmitter, but honestly that’s probably making it too easy to find. Better to have a whole host of slight anomalies.