Guardians of the Galaxy Vol. 2 is set after the events of Captain America: The Winter Soldier, so at this point, S.H.I.E.L.D. has all but collapse, and just left to the oversight of Coulson and a few of his loyal agents as shown on Agents of S.H.I.E.L.D.
So, although Earth is now used to unusual things happening, this would still have made headlines and seems to be a plot hole with the movie.
However, given how S.H.I.E.L.D. is now in hiding, it is quite likely that
Major (later Colonel) Brigadier General Glenn Talbot from the US military saw to the clean-up.
The incident was likely covered-up with a story of some kind of major chemical spill, especially given its limited effect and the small out-of-the-way town it impacted. This may also explain why there is no reference to it from the Avengers: Age of Ultron when the "incidents" are shown on the map, nor was it mentioned on Agent's of S.H.I.E.L.D. as having been dealt with by them. (Speculating)
Obviously, everything within the MCU is "connected" but there are often holes in the continuity, that are sometimes just best ignored to be fully enjoyed.