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.").

English is not my native language. I try my best to express my thought precisely. I hope you understand what I mean. If any misunderstanding results from culture gaps, I apologize first.