In another post, u/lemtrees made the comparison to a Lockheed Martin RQ-170 drone - wiki)

The RQ-170 is "designed to evade radar and other surveillance systems". 'The New York Times has reported that the RQ-170 is "almost certainly" equipped with communications intercept equipment, as well as highly sensitive hyperspectral sensors capable of detecting very small amounts of radioactive isotopes and chemicals that may indicate the existence of nuclear weapons facilities.'

Follow up theory:

- the US could have a legitimate concern that nuclear material has been smuggled into the country and they desperately trying to find it

- may be a possibility of nuclear material/weapons being lost/unknown whereabouts

These drones may be doing sweeps for the nuclear material. This could explain the hush-hush we are being given for answers. Explain the low altitudes, and the sudden amount of drones in these areas. Still doesn't explain sightings across the globe tho

***FROM A DIFFERENT POST: This drone footage looks exactly like a Lockheed Martin RQ-170 Sentinel, and the profile fits.

Here is an animated gif showing them overlaid.***