In Los Angeles Building and Safety does do door to doors in random neighborhoods and they also use ariel photos to find people who are building without permits. If it is obvious construction is going on from anywhere on the exterior, even from piles of lumber sitting in the driveway, it may not have been an individual reporting you.
Building permits are a huge source of revenue for the city and they have gotten much more proactive in seeking out any building without proper permits.
In the Santa Clarita area, building and safety's code violation officers are definitely doing a street-by-street search for illegal additions/conversions, largely due to an abundance of public complaints about overcrowding in houses with illegal garage conversions.
Regardless of how you were reported, you would need to disclose unpermitted additions or conversions when you decide to sell your property. It's best to get the required permits up front, rather than attempting to recover from the lack of permits after the work has been completed.
I had a client who was reported to the City Planning Department because of full expansion to his basement. We were quite amused and even had a laugh over it. We thought perhaps it was sour grapes because his house was done beautifully and was valued much higher than the neighboring houses --- and all the work was done with permits. So there!
If you are doing an obvious improvement, remodeling, expansion, etc. it could be anyone who is walking by, or one of your neighbors who is concerned about the integrity of the neighborhood, safety on the job, etc. Consider this a wake up call for you to go by the book, or get booked (fined!).
Doing it right the first time is always better than having to do it all over again (especially if the City makes you rip out the unpermitted work).