You pick a module that has 100% coverage and wonder can I safely refactor this?

Is this a rhetorical question? Code coverage does not imply 'behaviour coverage'.

Will the test suite catch if I make a mistake?

If you violate currently specified(1) behaviour: yes. Otherwise: no. In other words: Must assume 'no' unless proven otherwise. If the tests were developed using TDD methodology then you ~might~ be safe. But I definitely wouldn't rely on that. It's easy to change behaviour even without adding a condition, e.g. squaring a number introduces an 'invisible(2)' branch for positive and negative numbers.

(1) specified = there is a testcase for this
(2) invisible for coverage analysis