This is missing index request #1, and by default, this is the only missing index we’ll see by looking at the graphical execution plan. There’s actually a missing index request #2, which we can find in the XML (I know, it’s a little ugly to read. Bear with me).
I am of two minds on this. It probably should be easier to see multiple index candidates, but there’s already so much risk of people just copy-pastaing missing index recommendations that adding more seems like a bad idea.