An expense check? Could they have somehow gotten your acct. tangled in with an employees business expense reimbursement? That is all I can come up with?! Try to get in contact with their employee payroll dept. Have them cross reference the check there. You may just have the wrong department checking on it. Have other depts. cross-reference it also.
You can pretty well bet though, that an audit will find it!!

I am sure ccc's do audits quite frequently!
I would hang on to it, though! I wouldn't deposit it either, although I would be tempted!

Another suggestion...tear it up and toss it in the garbage! You know it isn't yours, you don't have to waste your time investigating their mistake. If they eventually trace it to you..tell them it has been shredded! You have already made your goodwill attempts to get it straightened out.
Hmmmmm, now that I think about it....maybe deposit it, if they never catch it...good for you...if they do catch it...tell them you thought it was a refund for an overpayment of somesort?! Be prepared to give it back, though!!

Just throwing darts in the dark, here!!
You are in a peculiar position. These are just some of my suggestions!! Hope others have more for ya!
takin' it one day at a time