Tethers are never visible to the enemy team.
Likewise ability telegraphs are not visible to the enemy team if they cannot see the player from whom the telegraph is sourced.
Scenario: Taric is in the open, his ally is in the brush. Taric casts Bastion on his ally.
The enemy can see Taric cast Bastion but cannot see on whom, nor any indicator as to its target's location.
Scenario: Taric remains in the open, ally remains in brush. Taric casts Dazzle.
The enemy again can see Taric channeling Dazzle and its associated telegraph; however they still cannot see Dazzle telegraphed from Taric's hidden ally.
To give a bit of back-end insight: If someone is hidden, they are hidden. No clientside data is provided for enemies currently invisible to a player; their whereabouts are completely unknown to your client application and so it is impossible for it to draw anything relating to that player or abilities cast by them.