Having a hunting knife concealed in the trunk of a car is not illegal under STATE law, because the only statute which prohibits the concealed carrying of knives is Penal Code section 12020, which prohibits dirks or daggers (and all knives are dirks and daggers) from being concealed ON THE PERSON. Although "on the person" can extend to things like bags or purses being carried by the person, carrying something in a trunk could not conceivably be on the person. A colleague of mine recently had a court rule that a knife in a backpack was not on the person, because it was too hard to get to if the hiker had wanted to use it. However, since California allows local regulation of knives, having a knife concealed in trunk coiuld conceivably violate some local ordinance.
Could you carry this knife concealed on your person? That is very unclear. The law regulating knives in California is presently in disarray. The only state statute which specifically prohibits knife possession is Cal. Penal Code section 653k, which prohibits the possession (concealed or not) of switchblade knives with blades over 2 inches in length. Of course, a hunting knife is not a switchblade.
The other applicable statute is Penal Code section 12020, which prohibits the concealed carrying upon the person of dirks or daggers. In the case of People v. Rubalcava, 23 Cal.4th 322, the California Supreme Court said that just about any pointy thing can be a dirk or dagger, including steak knives and knitting needles. The decision appears to say the concealed carrying on the person of any pointy thing violates the law, which would, of course, include a hunting knife. (The law also appears to prohibit the concealed carrying of a knife even in your own home, although it is legal to carry a concealed firearm in your own home. I love California law!)
However, the Rubalcava court also says that they are relying upon an older decision, People v. Grubb, 63 Cal.2d 614, which said that it was unlawful to have an ordinary useful object only in circumstances which indicated that the possessor intended to use the object for a dangerous purpose. Even with such circumstances, the possessor could defend by showing its intended use for peaceful purposes. It is very unclear whether the Rubalcava court is applying this same standard.
So, is the concealed possession on the person of an ordinary knife (not a switchblade) illegal? Frankly, at this time I do not know for sure. Penal Code section 12020 says that a knife is not concealed if it is in a sheath openly carried and affixed to the belt, but there are local ordinances which prohibit openly carrying knives (usually of over 3" or thereabouts).
This is an area of law in California which is completely fouled up at the moment, and it is difficult to advise anybody whether, and how, they can lawfully carry a knife, but I can say I don't think it is a violation of state law to have a hunting knife in a car trunk.
Depending upon the city or county, I am less sure about whether there could be a violation. Given the multiplicity of cities and counties in California, it is impossible to say for sure whether any prohibited knives concealed within vehicles.
Sorry I cannot be more precise than that at present.