The Sulaco probably transmitted it's data to the USCM HQ. I assume they must have decided to share it with WY (either willingly or via a crooked higher-up in the pay of WY).
Or perhaps WY simply intercepted the transmission and decrypted it themselves, who knows?
WY might not have gotten the Sulaco data first but they did get the data from Fiorina 161 first. The USCM most likely did not even know about the events of Fiorina. The Patna might also have been closer to Fiorina than any of the USCM's assets.
QuoteAnd the Tech Manual confirms that the Derelict wasn't destroyed in 'Aliens'
That was unfortunately retconned by Alien: Resurrection.
Quoteso it stands to reason that the USCM would have done some kind of investigation and sent another team to LV-426 to figure out what happened (even if it wasn't the particular events of Colonial Marines, which I'm not ruling out), which would constitute another Alien encounter.
No doubt they would send in another team to investigate. All that they would have found though is a big crater since all the Aliens and the Derelict were destroyed by the AP blast.