Guardians follow players under the effect of invisiblity potions with their eye and also sometimes turn around to face them, even though they don't attack/charge/flee.

Reproduce:

Go into survival mode, give yourself invisiblity and waterbreathing effects, find a body of water and spawn a few guardians,
You will see that their red eye follows you and they turn towards you, but they don't attack.
Make sure you don't wear armor or attack them, as they will be able to see and attack your normaly.