I live in Canada. In a few weeks, I will drive my parents to an airport on the other side of the border, drive back and then, later that day, pass through customs again with my spouse and kid. Will this look suspicious to the customs agents? Should I expect a more thorough questioning/inspection the second time?
There is nothing wrong, illegal, or out of the ordinary in crossing the US-Canadian border multiple times a day. People who live near the border do this regularly without hassle. I've done it personally, and I've relatives who have done it repeatedly on a weekly basis.
As you've explained in your question, you have perfectly legitimate reasons for making this trip. If the border officer is curious or suspicious, he can ask, and you can answer honestly.
- "What is the purpose of your visit?"
- "My parents have a flight out of Detroit. I'm driving them to the airport."
- "How long are you intending to visit?"
- "About 2 hours, just long enough to drop them off at the airport."
Most border guards aren't looking to hassle you, and if you're not acting suspicious and going about your legitimate business they'll usually leave you alone. As a Canadian citizen, you're permitted to come visit the US on a much less restrictive basis than a citizen of other countries. (Eg. no visa required for this sort of visit.)
Frankly the last time I did the six-times-daily-across-the-border trip from Detroit to Windsor/Ontario I never had a longer conversation with the border guards than "Hi/How are you/This weather is something else, eh?"
It should be no problem at all.
Note that any limits for what you can bring through customs are per day, not per visit, so although you can go in and out all day long, you can only bring the alcohol, tobacco, etc. allowances once per day.