He cast Locate Object from the moment they started the trip.  He has two regular Locate Objects, each 12 minutes, and one Extended Locate Object, lasting 24 minutes.  All have a range of [880 feet].  When one such spell runs out, there's not much point in starting it again until they're around [1500 feet] from where they were originally -- unless the whole trip is to last less than 48 minutes, in which case he should maintain solid Locating in order to not narrow the range he sweeps.

If they still have margin to delay the trip without running out of Location for sweeping the basic pathway, then it might make sense to try detouring into the largest caves.

Merrin is the one with mapping {software} and speed estimates; he's relying on her to say if they have the margin for searching caves at all, and how much margin, and then sure he can make the call on caves that seem particularly likely to have sucked in the Holding Bag and have more than 880 feet of radius to search.

He cast Locate Object from the moment they started moving downstream.  He has two regular Locate Objects, each 12 minutes, and one Extended Locate Object, lasting 24 minutes.  All have a range of [880 feet].  When one such spell runs out, there's not much point in starting it again until they're around [1500 feet] from where they were originally -- unless the whole trip is to last less than 48 minutes, in which case he should maintain solid Locating in order to not narrow the range he sweeps.

If they still have margin to delay the trip without running out of Location for sweeping the basic pathway, then it might make sense to try detouring into the largest caves.

Merrin is the one with mapping {software} and speed estimates; he's relying on her to say if they have the margin for searching caves at all, and how much margin, and then sure he can make the call on caves that seem particularly likely to have sucked in the Holding Bag and have more than 880 feet of radius to search.