If a user is logged in normally ,
there should be at least three cookies set :
ubbt_myid : USER_ID
ubbt_mysess : mapped to DB
ubbt_key : MD5(USER_ID + PASSWORD)
But when admin "Becomes" some member , the ubbt_key is not set.
I use this cookie to authenticate an UBB user (in my custom web app) , and found this situation.
ubbt_mysess expires when browser closed , but ubbt_key won't (if the user checks "Remember me on each visit.").