When attempting to drop a database user, you get the following message:
One place to begin is your stored procedures. The database user has likley been used in the “WITH EXECUTE AS” option in a stored procedure:
Hey, didja know that RedGate SQL-Search is FREE! Hint! Hint! Go get it. It will be a BIG help in this case.
Kirby L. Wallace 
This post was exceptionally helpful in 3 different ways. It showed me that there is an option to run a procedure or function as another user, that Red Gate had another ridiculously useful tool I could get, and I’ve got a solution for a problem that may pop up in future. Thanks!
There are limitations on the account that you can use. It cannot be a builtin account. “sa” is right out. Also, SERVER\Administrators, or SERVER\Users cannot be used because domain groups are a no-no. Can’t use anything like the NTAuthority system account. It must be a single user ID. And that user ID must be granted sufficient privs in the target database to accomplish whatever it is you have purposed it to do. Otherwise, you will find yourself staring down the dreaded “user so-n-so cannot access database ‘xyz’ under the current security context” (which makes me want to pull all my fingernails out whenever I see it).