Chap at work suggests the problem is the plastic bag (his job is to work this kind of thing out - something to do with moisture and planes falling from the skies).
He seems to think that the cache might in a warm place and as it warms up the moisture in the log book is taken up by the air, then when the temp falls at night it condenses out. The plastic bag traps the warm air in the bag. He said it's a similar effect to that of leaving potatoes in a sealed plastic bag. If there is a pencil in the film can that could also be a contributing factor.
He suggests:
1. drying to log book out in the oven, making sure the inside of the film can is dry and then popping the log book back in sans bag (and sans pencil). Then see what happens.
2. Stick a silica gel bag in the film can (has the added side effect of stopping trinkets from being stored in there).
Tsiolkovsky