Continuing from that question I asked you about what a borrowed chord was, another name it could be called is modal interchange or mode mixture. Still have any idea what it is?
that would be like if your song is in C but you use a chord from slightly outside of C. like Bb, Eb, or Ab, which are actually a part of a C minor key but not C major. using those chords in a Cmaj song is actually pretty normal and legit, and it is most likely the thing you are talking about