In general, income earned while visiting California and working during the visit is considered California-sourced income and is subject to California income tax. You would file a California non-resident income tax return form 540-NR. Technically, assuming there are about 260 working days in a year and you work 15 days while visiting California, then 5.77% of your work income would be considered California-sourced and subject to California tax. I don't have access to form 540-NR, so I can't determine if there is some minimum below which you won't owe tax.
Practically speaking, it would be very hard to California to track you down as long as your company doesn't list California on your W-2, and @Hal_Al is probably right that (mostly) no one actually complies. On the other hand, the statute of limitations doesn't start until you file, so if you never file, California could (in theory) come after you for the taxes at any future date without limits.
