I can find 37 occurrences of scomparso and 16 of sparito in the corpus. They seem to be practically synonymous to me, but I find only a few parallel translations using them interchangeably.
I indeed recall having encountered more sentences about missing cats etc. using “scomparso” than “sparito”.
I found an interesting discussion on this, which seems to suggest they are synonyms, but generally used slightly differently.
Basically it sounds like “sparire” is more informal, and generally used if you’ve (temporarily) misplaced something, like your keys, or if one of your friends disappeared for a couple of hours without saying anything, and you ask them “Hey, where did you go, you went missing earlier?”.
However, for cases of missing people, or (mysterious) disappearances (there is an example of a painting which disappeared from the museum without a trace, and of course the sentences with the cats we’ve encountered here), “scomparire” seems the preferred verb of choice.
According to ale96 there, “scomparire” can also apparently be used for a loved one who has “passed away”, whereas “sparire” would never be used for that.
Interesting! I’ve mostly used “sparito” to date. Ngram shows “sparito” more common, but only more recently:
Perhaps the advent of mobile phones has made people more scatterbrained
P.S. I just noticed it was comparing “Scomparso” with a captial “S” with the non-capitalised version of “sparito” in a case-sensitive way. Making it case-insensitive, “scomparso” does become the clear “winner”, though the 21st century trend in “sparito” persists.