"Two folks in their 70's, new car, what's the suspicion? who knows what the motive was?
it's all part of the mystery of travel, and you might as well get used to it."
The suspicion is that they might be smuggling people or drugs.
Border people used to pull over all the long haired young ones, until they realized that the couple in their 60s in the 50 foot motorhome could, would and was hauling weed by the tonne, while the stoner with a backpack had nothing. That is why most checks are random, in the abscence of any tips or paperwork issues.
I remember in the late 80s Heathrow switched from picking out likely suspects in the customs lineup, to just taking and searching every 25th person no matter what they looked like- and their drug interdiction rate went up sharply.......

