Please, please! It’s honestly not difficult to do.
I sat and watched the UFO’s behaviour…
See, in this case the apostrophe indicates an attribute possessed by the UFO! i.e. the behaviour.
The UFOs swarmed over the planet
This is just the plural form of UFO and therefore doesn’t need an apostrophe!
The UFO’s occupants swarmed over the planet
In this case the occupants of just one UFO are involved, but the apostrophe is needed to indicate that the occupants were associated with the UFO.
is wrong, wrong, wrong. If you ever see this then ask yourself: “The UFO’s what is being filmed?” The UFO’s lights? The UFO’s occupants?? The UFO’s cats and dogs?
The correct use here is simply:
as it’s presumably referring to UFOs in the plural so no apostrophe is needed.
The only time it can get tricky is when the plural form of UFO [UFOs] possesses an attribute, in which case the correct usage would be:
The UFOs’ behaviour
You KEEP the plural form [UFOs] but the apostrophe goes after the ‘S’ and not before the ‘S’, as it would if it was just one UFO on its own [UFO’s].