Thank goodness for this sign! There's a fork in the trail with path to the right heading uphill more or less directly towards Cathedral Rocks, but that's a dead end. The actual path drops off towards the left and then winds around the hill. I'm sure that they put the sign there after too many people took the wrong path to nowhere because it looks like the right way.
Update: I just noticed that both trails are on the Open Topo maps, and apparently the other trail does connect to Temescal fire road too on the other side of Temescal Peak. So much for my theory. But I'm glad I took the one that I did because it got me closer to the top of Temescal Peak.