Please keep an eye on the target's apartment from the van. If you catch no article sight of anyone leaving through the back door, notify me immediately. We cannot afford to be left in the dark about his movements tonight.

keep an eye on: This idiom means "to watch someone or something carefully" and uses the indefinite article "an" (because "eye" starts with a vowel sound).

catch sight of: This fixed phrase means "to suddenly notice or see someone/something" and requires zero article.

in the dark: When used metaphorically to mean "uninformed" or "ignorant about something," this idiom always takes the definite article "the".