Does TA implicitly call UseTrial() ?
No it does not. TrialDaysRemaining will just happily return the maximum trial days every time you call TrialDaysRemaining. Unfortunately this was a design flaw in the early versions of TurboActivate (the function should return an error instead), but if we fixed it now it would cause compatibility problems with customers using the functions incorrectly.
Short answer: always call UseTrial() before you call TrialDaysRemaining.