When a commercial airplane lands at an airport, is it always searched for suspicious objects?
At the end of the Airplane's day (It's last flight for the day) It's the mechanics and the cleaners' turn to be on the plane.
The mechanics start out by performing the daily check or overnight check, it's like a routine checklist to make sure that your airplane is well maintained and ready for another day of flying. They also look at the Cabin Log books of the aircraft for any maintenance issue the flight crew has encountered during the day.
These issues could be anything: ex:
"Seat at Row 28 won't recline properly"
"Sink is clogged"
and while the mechanics are resolving those cabin issues along with their daily checks, the cleaners also start their work. Cleaning all the seats, galleys and lavatories. They are not particularly looking for bombs, but they make sure to report any lost items they've found during cleaning.
Almost the whole cabin is cleaned up and inspected. So if anything is left, it will be found. Even the life jackets underneath your seats are accounted for, so if anyone decided to leave something under there, the mechanics would spot it.