>A pilot told me once that airliners have one or more locations where previously unknown to be on-board explosive devices suspected to be capable of going off in short notice shoud be placed, in case they are found during flight.
Yes. Actually, I believe the designated location is outside, on top of the left wing.
The least-senior (or most senior, and closest to collecting retirement - depending on the airline in question) member of the cabin crew is responsible for positioning the possible explosive.