I observed something stranger in his behavior.
I observed something strong in his behavior.
Do both of the above sound good? If not, how would you revise them? Thanks.
I'm not sure what it is you want to say here... strange or strong? (It's probably just a typing error.)
If you use the first one, you need to make it clear what it is you are comparing his behaviour to. If you aren't actually making a comparison, then use just the adjective 'strange'.
I think as Clare says that the first needs a comparison, but it could be used in answer to a question such as " Did you observe anything about him since you last saw him?"