2017-09-29 : If each target card is an illegal target by the time Deadeye Tracker’s ability resolves, the entire ability doesn’t resolve. Deadeye Tracker won’t explore.

2017-09-29 : If one target card is an illegal target by the time Deadeye Tracker’s ability resolves, the remaining legal target is exiled and Deadeye Tracker explores.

2017-09-29 : You can’t activate Deadeye Tracker’s ability without targeting two cards in a single opponent’s graveyard.

2018-01-19 : If a resolving spell or ability instructs a specific creature to explore but that creature has left the battlefield, the creature still explores. If you reveal a nonland card this way, you won’t put a +1/+1 counter on anything, but you may put the revealed card into your graveyard. Effects that trigger “whenever a creature you control explores” trigger if appropriate.

2018-01-19 : If no card is revealed, most likely because that player’s library is empty, the exploring creature receives a +1/+1 counter.

2018-01-19 : Once an ability that causes a creature to explore begins to resolve, no player may take any other actions until it’s done. Notably, opponents can’t try to remove the exploring creature after you reveal a nonland card but before it receives a counter.