In On a Comet
The splash was heard by no one. But astronomers helped them find it, deep in the ocean. When they pulled it out, it was not a meteorite. As a result, it was brought to shore and carefully hidden, to be analyzed by those with a "need to know".
The visitors had sent it after detecting radio traffic coming from the third planet. The next time around they expected a response.
It was good that there was time until then, as the comet swung around the sun and headed out to deep space, scheduled to return.