Doesn't matter. No one alive today will be alive long enough for there to be an answer. It was a 1 time observation. Whatever it was, happened a century ago. If it was an intelligent species that did it, it was capable of doing what we cannot do today, 100 years ago (more or less). Even if we were capable of crafting a response, it would be another century before it reached them. Likely it never would because where the origination is, has moved. Nothing in the universe stays in 1 place.
[beeep] it might have been 1 message that boils down to "I wonder what this button does" and then the world exploded
Leaving religion out of this, the odds of a more or less infinite availability of stars, with an infinite squared number of planets, and none of them supporting life is infinitely small.
BUT...
One of the things that many alien folks forget is that there are not 1 but 2 things that would have to be met for us to find another population of life. Both time and space must coincide.
People tend to think only of right now, because that's when they are alive. Looking at all probabilities there is the potential that there were other inhabited planets, within reach of us, but so long ago that there is no trace. And there is the potential that there will be the same, but so far in the future that the earth will have no trace of life. Then you'll have 'people' on our moon wondering "Are we alone in the universe?"
Likewise that in the entire universe our planet is the first to have life, and that it could also be the absolute last in the universe that will ever have life.
It's a lot like elk hunting. I know for a fact, that elk HAVE been where I AM. But for the life of me, I can't find them at all when I'm there
Time and space, both have to be 'there' at the same 'time'.
When you're thinking of infinite possibilities, all possibilities no matter how unlikely have exactly the same chance of happening. Fun stuff. Mildly depressing