[tinfoil hat on/seen too many James Bond movies]
Foreign Country Taxi Driver: Where to?
You: Airport
TD: Do I need to hurry, what time is your flight?
Y: Plenty of time
TD: Going home today?
Y: Yes, can't wait to get home
TD: Pushes button under dash, buzzes boot.
Meanwhile back in the boot out of your sight: Next to your bag is not a spare tire but Mini Obama, the vertically challenged terrorist bomb maker. Who now knows you are going on an international flight and can set the timer correctly and place it in your bag. The 30 to 60 min ride to the airport is PLENTY of time to accomplish this.

Your bag was out of your control and out of your sight to person(s) who know your travel plans. Every pax who ride a taxi to the airport checks in a bag that was out of sight.

[/tinfoil hat on/seen too many James Bond movies]