Definitely a missed opportunity. S2 needs to play for the euchre. That means leading the boss trump on 3rd street to hopefully clean out the maker and take it from there. If that lead doesn't clean out the maker, I.E. the maker called with 3 trump and thus has 1 trump left after S2 tries to clean him out, S2's lead still can create chaos on 4th street or 5th street as S1 may not have a boss offsuit to play--that's still a win for S2's team. Chaos is good!
That said, the maker is an aggressive player. He will be calling with just 2 trump a lot in that spot. S2 needs to exploit that fact. And those times the maker calls with 3 trump, and thus has 2 left on 3rd street, S2 leading garbage offsuit isn't likely to lead to a euchre anyways, so S2 should just go ahead and attack the part of S1's range that just called with 2 trump and clean him out.
An advanced hand-reading aside: Given that S2 decided not to try to clean out the maker, he still led the wrong offsuit (KS) on 3rd street. S2 needs to pay attention to his P, S4's actions and know how to decipher them. The instant S2 trumps in on 2nd street, S4's job changes from keeping his guarded kings/queens intact as long as possible to prevent a 2 point march TO keeping his suited ace intact should he have one to give his team the best chance at a euchre later in the hand. What this means is when S2 trumps in on 2nd street, and S4 throws off a spade, he is actually communicating to his P that he is NOT covering spades since if he had AsXs he would not break it up. This is a very difficult concept to grasp. I would estimate less then 10 players in the world can handread at this level. So When S2 sees that signal, he should lead his club instead of the spade.
But better still, S2 should lead the boss trump to clean out the maker, and look what will happen: S4 will play the AC on that lead signaling to his P he has clubs covered, and then the euchre is inevitable.