As a rule of thumb, you can get a so-called "bare noun" when the noun represents a collection of indeterminate items. So for example, consider the pair:
(a) I saw some children playing and some adults going to work.
(b) I saw children playing and adults going to work.
Case (a) uses "some", which you may be used to considering as an 'indeterminate' article. In case (b), no article is used at all.
Now, although "indeterminate", the crucial thing about using 'some' in (a) is that it implies that the children/adults could be specified. In case (b), by using bare nouns, you imply that the children/adults are basically unspecifiable. It would sound a little odd to say, for example:
"??I saw adults going to work. I recognised them as Peter and Jim from
whereas it would sound more natural with the article:
"I saw some adults going to work. I recognised them as..."
Now, this rule of thumb extends to various "set phrases" where it is unusual to specify the particular item in question. For example:
I travelled by plane.
Here you don't generally care about the specific plane. Notice that if you do, re-wording in a way that uses the article becomes more natural:
*I travelled by plane whose flight number was 731.
I travelled on a plane whose flight number was 731.