It's a long shot, but does anyone have any info on departures from YXU (London Ontario)? - this is a tiny airport, with only 3 US departures per day and no airside facilities except for a vending machine. Friends of mine (family of 4 with small kids, non English speakers) are on YXU-DTW 3176 tomorrow. Delta.com advises arriving 3 hours in advance for inbound travel to US. My gut says there is no way they need anything like this much time, but I'd rather have some information to go on. Any advice or experience (based on other small canadian airports perhaps)?
Call the airport and ask them. With smaller airports like this you will probably reach someone in management fairly easily. Assuming they clear CBP in DTW you are probably right but better to be safe than miss the flight.