They ain't randomly looking at phones, if they do go looking for your phone its because they think/know they will find something. I don't know of anyone who has been searched or refused in a long long time.

As preclearance is not the US you have the right to decline any question or search and ask to be returned immediately to the public part of the airport. This cannot be refused.

I have a push from my boss to use preclearance when travelling to the US, it keeps the stress levels down but its not like I wouldn't use preclearance, its the best way to go