.odt docs being mysteriously locked requiring passwords

can the word document .odt files password protect themselves or does it mean intruders are doing it? This happened after passwords were removed- and doublechecked that they were removed- and new unknown passwords later apparently were put on.
How can this happen, and is there a solution or software to remove/identify passwords.

Maybe this link could be a interesting reading. how-secure-is-libreoffice-when-using-password-protect