I'm trying to learn music theory via bass tablature. I started by looking at Muse's Muscle Museum and translating the tablature into notes, and trying to define more music theory information about the song from my findings.
I noticed the song is in the key of F# minor, as it contains all of the constituent notes in the key of F# minor (F♯, G♯, A, B, C♯, D, E).
I was confused to find that in the song, during the transition between verses, the bass riff contains an F note. I was confused by this as the F note is not in the key of F# minor.
Under what conditions would this be allowed in music theory? How is this concept defined theoretically? By my count, there are additional notes in the scale now,(the F). I'm curious if I'm over-thinking things by suggesting there's a theoretical convention for improvisation like this, or if there is in fact a name for techniques like this.