I have realized that not all chains and double bonds are numbered. I have noticed that up until 3 carbons in a chain, the double bond is not numbered, but I am not sure about the chain.
When there are 2 carbons in a chain, the double bond will be at the number 'one' position. Therefore it's not always mentioned in the naming.
For example: Ethene It can be written as 1-ethene or eth-1-ene. Since the position of the double bond can be at no other places than the first, the position in the naming you saw, wasn't mentioned.
When 3 or more carbons are present, the double bond can be between any 2 of the carbons, ie. either the first position or the second. (but-1-ene/butene or but-2-ene)
Position of double bonds and that of branches, must always be mentioned while naming organic compounds.