Find a username within instance wide objects
If it's in the text of a proc/function you could use the easy and free to use third party plugin of redgate:     SQL Search, there are also ways to do this with TSQL, see answer on stackexchange searching through procedures

However, very likely your proc will fetch the value from a table that stores it, so the proc doesn't have to be hard coded. 
If you want to search through a bunch of columns at the same time, this link might be useful to you:
Finding string in different tables