Take your suggestion. Thanks, David.

Thank you for your research, Mark. I would not object to either "I wonder if he noticed anything", or “I wonder whether he noticed something?”, using simple past tense rather than present perfect, but in the end I much prefer breaking it into 2 sentences like this:
"I wonder. Did he notice something?"

This is Johannes speaking. I asked ChatGPT. The reply: “Do you suppose he has noticed something?" And in unit 141, Kaisa says: "What should he notice?" Somewhat similar to your suggestion, except with "I wonder", it's Johannes asking the question to himself, while in ChatGPT's suggestion, Johannes asks Kaisa what she thinks. I asked ChatGPT: How about: 'I wonder whether he has noticed something?' The reply:
Yes — that works very well and sounds natural in English.
The nuance is slightly different:
“Do you suppose he has noticed something?” → a direct question to another person.
“I wonder whether he has noticed something?” → more inward, reflective, or tentative.
The Norwegian “Mon han har merket noe?” has a somewhat thoughtful/speculative tone, so your version is arguably closer stylistically in literary prose.
You could also consider:
“I wonder if he has noticed anything.”
“Could he have noticed something?”
“I wonder whether he noticed anything.”
The choice between “something” and “anything” depends on context:
“something” suggests the speaker suspects there is something to notice.
“anything” is slightly more neutral/open-ended.
Any further thoughts?

Earlier in this chapter I translated "mon" as "I wonder" - unit 99. Now I double-checked in my copy of McKay's Modern Norwegian - English dictionary and found it there under "mon" as well - "I wonder (whether, or if), I should like to know (whether, or if)."
So for this unit I would suggest "I wonder, has he noticed something?"
(I wonder where the Norwegians got this word;-)