I don't know about Canada, but in the US, Apple charges a flat rate repair price for liquid damaged units, shipping them to their depot facility, of about $1240 plus tax for MacBook Pros, $750 plus tax for MacBooks (and Airs I believe). This is based on their highest level damage rating, Tier 4 damage. If the coverage you purchased was AppleCare, it's important to understand that it is not insurance - it's an extended service agreement, designed to cover failures that were not the result of physical damage.
As far as the moisture indicators being triggered by changes in temperature, if you're talking about walking from your dorm to your classroom, odds are the change in temp won't generate enough condensation to trigger the sensors. However, it's possible if you leave your computer in the car overnight in a very cold, moist environment, and then bring it into a very warm room, that it could create enough condensation... much like a cold drink on a hot day. That being said though, the simplest answer is most often right... if you're in a college environment, your computer could have suffered a liquid spill while it was out of your sight or out of your control.