If there has been no communication or contact, then we (the authorities ) cannot possibly know for certain that the observed objects/phenomena are extraterrestrial alien artifacts. All we have is some list of anomalous observed phenomena.
We've mistaken natural phenomena for the deliberate actions of way-more-advanced beings from the sky before. The Zeus theory of lightning and thunder was once pretty popular.
I would go ahead and disclose our ignorance of the cause of the new phenomena, rather than disclose an unjustified guess about their cause. Of course, many segments of the public will speculate that they're really aliens, but others will jump to different conclusions; most popular among these will be that they are deities or other agents from a spiritual realm, or that they are only a hoax by the government to distract everyone from the latest trivial political scandal.
Other possibilities include elaborate sabotage of our detecting equipment, an unknown natural phenomenon occurring for the first time, glitches in the matrix because it turns out the "we're all in a simulation" crowd were right all along, or mischievous young pranksters from the Wizarding World.
All these might seem to be poor explanations, but given observations that all of our past theories say are physically impossible, there are no immediate "good" explanations, and "aliens did it" is at best no better than any of the others on the list.
One very likely long-term outcome is that the new phenomena stop happening as suddenly and mysteriously as they started, and we never do figure out what they were. (If they were indeed aliens, that might also be the best-case outcome.) If that happens, disclosure or not-disclosure would make little difference in the end, but a false or unjustified "disclosure" could have very bad and unnecessary consequences.