Indeed around the time of the unlock (yesterday 15:22 UTC) there has been also an earthquake (M 5.6 - 9 km W of Sulusaray, Turkey, 15:11 UTC)

The unlock was likely due to it, as reported by A.Magazzu in https://logbook.virgo-gw.eu/virgo/?r=64020.

This makes difficult to disentangle if there has been aslo an impact from the helicopter.

I've checked with some Q-scans if the transit of the helicopter caused glitches in Hrec.

Figure 1: During the first transit at 14:46 UTC no excess noise was visible in Hrec, while in the CEB microphone is clearly visible the sweep caused by the helicopter.

Figure 2: during the transit at 14:59 UTC some barely coincident noise is visible in Hrec and the CEB microphone. The frequency is not exactly the same, so this may have just happened by chance.

Figure 3: some excess noise but no evident correlation at the time of the transit at 15:11 UTC. Nothing evident during all the other passages of the helicopter.

This site Elian suggested: https://www.flightradar24.com/ keeps records of aircrafts tracks.  Specifically, this is the helicopter that was flying near us on April 18 and 19: https://www.flightradar24.com/data/aircraft/mm82053.  Position and time are provided,  so is possible to locate its position at a particular time. Data are kept for 7 days. In particular its track of April 18 afternoon (attached shapshots) maybe passed a bit closer than 2000ft from CEB and NEB.

Nice idea. However the original restrictions - still enforced, as reported in the attached text of the forbidden areas of the latest release of the AIP publication (https://www.enav.it/servizi-online) - are cylinders of 2000 ft = 610 m of height but radius of 0.3 nautical miles = 1823 ft = 556 m. So probably the helicopter respected the NOTAM. 

" So probably the helicopter respected the NOTAM. "

I guess not

The altitude was also very low at the closest point (400ft --> 120mt).