107.3b

If a player is casting a spell that has an {X} in its mana cost, the value of X isn't defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0.

Isochron Scepter and Unexpectedly Absent.  The only legal choice for X is 0 when you cast Unexpectedly Absent through Isochron Scepter.

This doesn't apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, "Casting Spells."

Thornscape Familiar and Unexpectedly Absent. Yet it needs only WW, you can cast Unexpectedly Absent X = 1.