Can anybody explain why in this case it uses "zien" infinitief instead of "gezien" past participle?
Thanks for the reply! This is very interesting. It makes sense, so heb in this case is used as a modal verb instead of the classic auxiliary role that it plays in the present perfect structure. Or am I off?
No, not quite. "Zien" is used as an auxiliary verb to the main verb "flirten". It's a so-called transitive auxiliary verb.
In Dutch, you can't have a past participle function as an auxiliary verb. Therefore we use the infinitive "zien" instead of "gezien", even though the sentence is in the present perfect. You can only tell it's present perfect because of the extra temporal auxiliary verb "hebben". See also this link.
What if one wants to say "to hit on sb"? That's a bit stronger than just flirting. Is there a direct translation of this expression in dutch?