(two people talking about a third person)
A: There is something strange about him.
B: ----.
A) I noticed that
B) I have noticed that
Which one would you use?
I think you can use both. The simple past indicates some kind of detachement about the fact (Yes, I noticed that some time ago...). The present perfect can show that the person's ready to discuss the "strangeness" at length.
Yes; "I noticed that [some time ago/as soon as I met him/etc.]" or "I've noticed that too".
MrP