So I was thinking, if I rode a coworkers bike home and returned it early the next morning before he noticed, is that legally considered stealing?
Would this be legally considered stealing in CA?
It wouldn't quite be theft; California theft, like theft in many places following English legal tradition, requires intent to permanently deprive the owner of property. However, it fits section 499b of the Penal Code to a T:
499b. (a) Any person who shall, without the permission of the owner thereof, take any bicycle for the purpose of temporarily using or operating the same, is guilty of a misdemeanor, and shall be punishable by a fine not exceeding four hundred dollars ($400), or by imprisonment in a county jail not exceeding three months, or by both that fine and imprisonment.
In the UK this is covered by Taking without consent which reads:
12 Taking motor vehicle or other conveyance without authority.
(1)Subject to subsections (5) and (6) below, a person shall be guilty of an offence if, without having the consent of the owner or other lawful authority, he takes any conveyance for his own or another’s use or, knowing that any conveyance has been taken without such authority, drives it or allows himself to be carried in or on it.
There are many "levels" of theft in California, but I always begin with "theft is taking property that without asking and without intent to return". Stealing a bike would most likely be considered petty theft, stealing a possession valued under $950. One possible argument to defend you may be lack of intent to steal; http://www.gddlaw.com/criminal-defense/theft-crimes/