Short NEXUS grocery/gas/Trader Joe's trips from Canada south (to the US) are quite common to both CBSA and CBP. I did a round-trip to Blaine today to collect mail and fill my gas tank. I was back in Canada in under 20 minutes with only routine questions.
Short trips NORTH and back are less common and are greater scrutinized for drug reasons. Extenuating circumstances (like using rental cars) may increase the visibility.
I had a good friend with a very high US national security clearance cool his heels waiting while the CBP tore apart his rental car because a dog signaled on a routine sniff.
It can happen to everyone.