Okay, I'm back. The sensors are not on ALL signs, just some.. generally the ones that make the most sense. Excuse the lousy pictures from my phone..
This one is at the bottom of the stairs at VCC in the down direction:
I noticed lots of people walking down the exit stairs to the platform at VCC where there is not a sensor. It was quite late at night, but I imagine that human traffic will cause people to use the correct entrance during the day.
VCC exit stairs:
At Waterfront station, the Granville Street exit AND entrance both have sensors for some reason.
Granville Street exit at Waterfront Station:
The Waterfront station door has sensors across the whole width, however, so who knows how accurate it may or may not be.
I entered the system at Waterfront and didn't notice the sensor (was looking near the Fare Paid Zone signs, but that is not where they are.. they put them where the last sign is before stepping on the platform). Didn't see it at first as the outbound train was sitting there, so I rushed on and looked around at VCC where I didn't see one either as I was looking for the Fare Paid Zone signs again. I asked an attendant who wouldn't tell me. Kind of rude an evasive rather than just saying 'sorry, can't say'. He proceeded to watch me look around, shortly after I went down the stairs I found them.