I live in downtown SF, I often don’t lock my door even when I run out to do quick errands and frequently forget at night. I live in apartment building with a security door but people do get buzzed in and I have never had an issue.
In fact this morning we overslept for a flight and my boyfriend left to go get coffee for the cab ride and both of us kept forgetting things so went in and out of the apartment before we got in the cab. When our plane landed I asked him if he locked the door because I didn’t think I did and he couldn’t remember either so we texted our building manager and sure enough we hadn’t locked it! I do feel better that she locked it with her master key because we will be gone a few days but it probably wouldn’t have been an issue.