Under ideal circumstances, the Tribes keep track of all offspring born to their members and kin, and after each birth perform the Baptism of Fire on the babies or pups. This Rite identifes any of those children who have bred true (who will grow up to become Garou), and when this is the case it binds a Gaffling spirit of the Tribe to them, known as a kinfetch. Once the child grows towards puberty and their First Change draws near (not necessarily imminent, there may be days or weeks to go), this kinfetch hunts out the nearest adult Garou and leads them to the cub. The Cub's Tribe (and sometimes their Auspice) is identified by the kinfetch.
Lost Cubs result when some step of this process goes wrong.
Most commonly, the Garou lose track of their descendents for one reason or another- Garou die, kin move away and lose touch with their Garou family, a male Garou doesn't reailise that he has caused a pregnancy and moves on, kin die and their children are moved into care by the mundane human authorities or adopted by non-kin. When eventually these circumstances lead to the birth of a Garou child, nobody performs the Baptism of Fire, and the child has no kinfetch. A Lost Cub may also be the result of the kinfetch getting lost or meeting a sticky end on its way to alert the Garou- kinfetch are weak spirits and usually not very intelligent. Other scenarios are also possible- a failed Baptism, for example.
Such Lost Cubs are potentially in great danger, and are themselves a danger. Nobody will recognise their impending First Change and a Veil breach becomes worryingly likely when they finally shift for the first time. The cub-to-be may also be sought out by other supernatural forces once their Garou nature becomes clear. They may be taken by the Black Spiral Dancers to join them, ghouled by a Vampire, or any number of other unpleasant possibilities.
The 'lucky' Lost Cub who is found by the Garou has an uncertain start to their cubhood. Traditions vary between Septs- in some, the first Tribe to find them can Claim them. Others may insist that an effort is always made to trace their 'correct' Tribe, but the child's Tribal bloodlines may be so mixed or diluted that even if their ancestry can be determined, there is no Tribe with a clear claim. Some Septs may allow the cub a degree of choice, in others the cub may have none. In some Septs, anyone may Challenge for a Lost Cub. At the Hidden Walk the first Tribe to call dibs claims the cub unless there is good reason to think that the cub is of a particular Tribal bloodline, when the Tribe of the bloodline in question may expect or demand a say. Any other Elder may then challenge that Tribe for the cub. This process is not, however, set in stone, and there has been more than one case of cubs being offered the protection of one or more Tribes while deciding for themselves where they think they belong.
Even if Claimed by a Tribe, a cub is not officially of that Tribe until they have passed their Rite of Passage and been accepted by the Tribe totem. It is not impossible for such a Lost Cub to find during their Rite of Passage that they belong in a tribe other than the one that sent them on their Rite.