Some words seem to have opposite meanings in different contexts.
scan, have both the meanings of
look carefully and
How to identify that?
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You just have to look at the context.
Scan can have the former meaning,
to look carefully, as in:
The detective scanned the scene for clues.
Or it can take your second meaning
to look carelessly, as in:
I scanned the magazine, but didn't find anything interesting.
There are times when the precise meaning might be ambiguous. Consider:
The detective scanned the scene, but didn't see anything out of the ordinary.
Did he do a careful scan, or a superficial scan? It's impossible to say without additional context.
The key is that scan really just means "to look over," and thus scanning can be done thoroughly, carefully, superficially, or in any number of other ways.