What they don't know

He nearly cried out at the impact. The Black Paladin dug his hands into the fabric of his shirt, pushing him harshly against the wall, "Where is Lance?!" Keith snarled. It really was remarkable how his eyes changed to resemble those of pure Galra. It would have scared him if his introduction to the guy hadn't been a gushing retelling of a 'bonding moment' between him and his brother.
Alec merely rolled his eyes. "Haven't you been listening. I. Don't. Know. And to think Lance said you were smart." Keith was pratically growling as he pulled his arms off of his shirt. "I'll say this once. You will either help me find my brother and fix the mess you made or you will let me leave with that ship and stay the fuck away from him."

In which Lance goes off on a solo mission without the others knowing only to be left behind when the team disappears and having to deal with the aftermath.

(They do get reunited. In fact most of the time lance spends a part from them in shown in
Lance's flashbacks leading up to their reuinion.)