Invisible notes

I downloaded a Musescore file and deleted the second voice from it. My file enigma.mscz is in MS3-format and contains just one measure. As you can see, it contains just 4 notes in treble, but when you play it, you can hear also bass notes, which are invisible in the score. What are these and how could I edit or at least see them somehow? Thank you in advance and Happy New Year.