The sefer ביאורי אונקלוס here says that the word פּוֹן indicates that the action or the matter is a possibility and not a certainty. Thus, Bereishis 26:10 means "one of the people might have lain with your wife", and this is the Targum's translation/explanation of the word כִּמְעַט.
But on Bereishis 31,27 he writes that it says in the Mesorah that there are five places in the Torah where the Targum Onkelos writes the word פּוֹן even though there is no corresponding word in the posuk, and this is posuk one of them. Because when the posuk says וְלֹא-הִגַּדְתָּ לִּי, וָאֲשַׁלֵּחֲךָ בְּשִׂמְחָה it does not mean that Lavan said to Yaakov "If you had told me I might (פּוֹן) have sent you away with joy" - it means "I (certainly) would sent you away with joy", and so the word פּוֹן should not be here in the Targum.
However, Lavan could not have been saying that he was sending him away with a complete heart, with a heart full of joy, because Lavan had already indicated by his chasing after Yaakov in order to forcibly make him return that Yaakov's leaving him had angered him very much. Therefore, the Targum added the word פּוֹן to clarify that what Lavan was really saying was "for the sake of appearance I would have sent you away with joy and songs" as if it was genuine, but it would not have been done with a full heart.