The author's excellent 'powers of observation' [=ability to notice and pay close attention to things] are evident in the book's detailed descriptions.
The plants seem to have recovered from the disease, but they are still 'under observation'. [=someone is watching them carefully]
They're keeping him at the hospital 'for observation' [=so that doctors can watch him carefully] for another few days.
(..) Go! FOR A COP'S HIDE Monsieur Choucas! Did you notice that I said Monsieur Choucas instead of Inspector Choucas? That was an astute observation. Very astute. And all in order to say that you don't have a badge anymore. And if you have no badge then you have no right to carry weapons. (..)1981 To Kill a Cop