Would a single sink full cause a contrail 400 ' long " ?
Most assuredly. Water tank head pressure decreases at altitude ( think of the number of times there is no water in an upper deck galley in cruise ) and a marginal faucet that depends to some extent on system pressure to shut it off fails to do so, you have a non stop flow down the sink and out the mast.
Not an every day event but flights have definitely depleted a complete water supply via a faucet that fails to shut off and for whatever reason, goes unnoticed.
PS: I think Boeing were fibbing
