I think it might have just been some of Mike's papers from the firm. Remember when Mike told Trevor he could lend him some start up money, and Trevor said "How"? I think this is going to lead into Trevor finding out Mike is a Lawyer and possibly use it to black mail him. The next episode preview said something about Louis knowing Mike's "dirty secret" and him needing to fix the whole not having a real Harvard J.D. issue. My guess is Trevor tells Louis maybe to get back at Mike?
About him losing the briefcase, I can see it. When you get into a fight/get that riled up your mind can get pretty side tracked. I did wonder why he didn't reflect on it later, though.