How did David explain why he is alone?
Malbim: The king commanded that no one else know even to where he goes.
What is the meaning of "Yodati"?
Rashi: I exerted them [to go elsewhere]. This is like "va'Yoda Bahem Anshei Sukos" (Shoftim 8:16) 1 .
Radak: This is like Hodati; I informed them where to go; I will meet them later. Perhaps this is a four-letter root.
There, Rashi explained like the Targum, he broke them (like he threatened "v'Dashti Es Besarchem"- Shoftim 8:7)! This requires investigation. (PF)
Where is Ploni Almoni?
Rashi: It is concealed; it is Almon (a 'widower') without a name. I do not want to mention it, for it is concealed. Also Ploni is like "Ki Yipalei" (Devarim 17:8), i.e. concealment.
Radak: Almoni is like Ilem (Shemos 4:11, a mute), i.e. a place that is not spoken about.
Malbim: It is an unknown place, lest someone who was in David's place will go there and tell the youths.