The term UFOacronym for unidentified flying/flying objecthas become a common noun, so it is appropriate to write it entirely with lowercase letters.
However, in the news related to the tensions between the United States and China, it is common to find phrases such as “Are there UFOs of extraterrestrial origin over the United States and Canada?”, “US intelligence does not rule out that the third object shot down is a UFO” or “The reactions and memes left by the shot down UFOs”.
Although originally this voice was an acronym, an acronym that can be pronounced as a word, it has already been incorporated into the common lexicon of the language and today it is written with all the letters in lowercase. This process is very frequent in this type of acronyms (sme, laseretc.), which, in their evolution, go from being written in capital letters to doing it in lower case and This is how they appear in the academic dictionary (UFO, sme, laseretc.).
Regarding the plural, it should be added that, while they are acronyms, it is appropriate not to pluralize the term itself in its written form, but only the determiners and adjectives that accompany it (SMEs). However, when they are already lexicalized, when they become common nouns, form the plural according to the general rules of Spanish (ufosSMEs, lasers).
Thus, in the previous examples, it would have been preferable to write “Are there UFOs of extraterrestrial origin over the United States and Canada?”, “US intelligence does not rule out that the third object shot down is a UFO” and “The reactions and memes left by the shot down UFOs».