I had another incident yesterday where an overtaking cyclist doesn't bother to announce their presence.
As a result, I was in the middle of a complicated intersection looking over my shoulder at what was making the leaves crinkle so close behind me.
It turned out to be a fully decked out, experienced looking commuter, so I called out to him, "Hey, let somebody know you're back there!" and he replies with a "hi" and cycles
along unconcerned.
This isn't the first time something like this has happened to me and I find it irritating and, sometimes, as startling as when cars drive by and someone yells out to startle me. Frankly, a majority of the time riders don't say anything, I notice the sound of the bike before I'm aware they're behind me.
If you're new to riding, please remember to let someone know before you overtake them, that you're coming up on them. If you are an experienced commuter, please let someone know you're overtaking them so as to have a safe passing. Not everyone commuting is pretending to be in a race with you.
Yes, I do have a mirror, but there are plenty of times when you don't use it because you're, appropriately, paying attention to what's going on in front or around you.