He should have subconsiously sensed Sidious through the Force and sensed his intentions.
This only proves that Sidious was far more powerful then his master, if the master couldn't sense his apprentice's intentions he is weaker then the apprentice himself. There are 3 things that notify that Plagueis was weaker then Sidious.
1. He slept and his apprentice figured out where he slept, thus he failed in hiding and an even bigger failure that he had his guard down
2. He taught Sidious everything he knew(save for immortality, but that is irrelvent as Sidious figures out a way to achieve this anyway.)
3. He couldn't sense what his apprentice was about to do, this was clearly the biggest sign that Sidious was more powerful then his master. Force sense to the jedi/sith are pretty much like breathing, the moment you can't sense another's intentions you are lost and you will die.