"casually observed" is correct and usable in written English.
It is typically used to describe something that was noticed or viewed in a relaxed or nonchalant manner. For example: "As she was walking through the park, she casually observed the birds flying overhead." This implies that the act of observing the birds was done without much focus or effort.
Attlee was so far from being a passionate ideologue that his wife Violet once casually observed: "Clem was never really a socialist, were you, darling?
The use of this weapon in this conflict should not surprise anyone who has followed the war or even casually observed the Syrian military over the years.
Many of the scenes -- Carlotta awkwardly entering a swimming pool for a group therapy session, Alicia forging a child's drawing in a steamy bathroom, Fred lolling on the kitchen table -- appear casually observed, with an almost documentary open-endedness.
As the taxi driver that took me to the march casually observed: "If you are not on TV, you do not exist".
However, these assumptions do seem reasonable based on the provider relationship between the two systems and based on what we've casually observed in the performance of our own systems.
I glanced at it and casually observed that it didn't list the doctor's phone number.
Then I saw a young man strolling casually, observing the traffic and generally taking in the scene.
I like a movie that the audience actively has to participate in, and not just casually observe.
In this case, petitioner, by securely wrapping and sealing his package, had manifested a desire that the public not casually observe the contents.
As a character casually observes early on, we might remember this seemingly insignificant evening later in life as an anecdote that begins: "Once upon a time in Anatolia…".
They soar effortlessly, inches off the wave tops, accelerating out of the troughs, and casually observe strangers with glances similar to those stolen by passengers in an elevator.
