Cosier is right that Second Chances has to do some locking in of eligible targets when it is enqueued. If some units leave play in a way eligible for Second Chances, then immediately some other unit leaves play in a way that would be eligible for Second Chances while Second Chances is waiting around to do its thing, Second Chances would not help the latter unit no matter what.
There’s no other ability in the game that works like this because no other ability in the game tries to do what Second Chances is doing. Normally if there’s a triggered ability that affects a certain thing, it locks in what it affects when it is enqueued, and normally if there’s an ability that lets you choose, it lets you choose from a broad class of things during resolution.
Second Chances says “Whenever one of your non-token units leaves play from something other than combat damage, return it to play. Once-per-turn.” I am interpreting this to mean something like “Whenever one or more of your non-token units leaves play from something other than combat damage and this ability hasn’t triggered this turn so far, return one of them to play.” It seems like that ability locks in the set of eligible units (“one of them”) when it triggers.
For Cosier’s concern about choosing units that aren’t there anymore, let’s again think about what we would do if the card said “Whenever one or more of your non-token units leaves play from something other than combat damage and this ability hasn’t triggered this turn so far, return one of them to play.” With this text we have to choose the unit (not decide at random) during resolution. If we locked in a set of {Skeleton Javelineer, Thieving Imp} when we enqueued it, but the Javelineer was gone, we would still have to choose something. This is true in the same way that a player cannot choose a target for Orpal’s maxband that was around when the maxband triggered but is not around during its resolution. I think that choosing randomly vs. not choosing randomly should not change that.