They are likely attempting to determine whether the module has some degree of protection from the rain. I created a protective cover out of a plastic soft drink bottle because both of my modules are directly exposed to the weather (see my post couple of pages back).
Rubber gaskets provide excellent moisture protection for these modules. The electronic board is shielded by two gaskets, one on the battery lid and the other hidden at the bottom of the device. The temperature sensor and moisture sensor are the only parts of the board that are naturally exposed to the outdoors, and occasionally moisture can enter the board through those openings.
My issue boards had no interior rust or mildew, no evident moisture penetration, and they both started to devour their batteries rapidly after about a year. This suggests that some components age with time and result in this issue. I believe a capacitor or an oscillator is to blame because resistors typically don't deteriorate over time.
By the way, I measured my boards' current usage and discovered that they use roughly 20mA, which is obviously too much. At that load, AAA batteries will survive for roughly 40 hours because they have an approximate 800 mAH rating. In fact, my devices' batteries run out in about a day or two.
The best solution I could come up with in terms of cost (about $15 in components) and longevity is my solar cell solution, despite the fact that it is not particularly elegant and should not have been required in the first place. One benefit is that I no longer have to bother about batteries at all. Always keep NiMH batteries between 80% and 100% charged. They shouldn't need to be recharged or discharged frequently at that level, thus they should last a very long time. Like I did, Netatmo should upgrade their design and add solar cells to their exterior modules.