If you decide to side with Vernon Roche you invade Loredo's Tower. On the second floor there is a bunch of guards sleeping and a locked chest. For some reason my Journal tells me that I found a key in it, which I pocketed since I didn't know what it was for.

I did no such thing according to my inventory.

Anybody know how to open that chest? I took care of everybody, I looted everything not nailed down, pried loose what was and then looted it and still zilch. Any idea?

i had the same problem

have you started the mystic river quest and put the mail in the royal mailbox?

yes i did

i did all quest and picked all loots but still there is a locked chest in guard's room

in my first play trough i had sided with Roche and had the same problem, i could not open the chest it kept telling me you need the key and i looted everything i could in the whole level

There are two locked chests. One on the first floor at the door entrance and one at the second floor near a guard sleeping.The key opens the locked chest on the first floor.

No idea how to open the locked chest on the second floor.