Hi Sam. 

I don't work with Audiomoths but we deploy a lot of field equipment. That looks like a water breach. Audiomoth enclosures have gaskets that should keep them waterproof, however if you have a heavy downpour, water will penetrate any small seam it can find. If this is the cause, one thing you may want to consider is to tape the seams where the enclosure opens and closes. We use electrical tape on our seams to add additional water resistance.

The water could also be due to condensation. If they're deployed in a humid environment and especially if it gets cold at night, you can get a lot of condensation. It looks like the water gathered in one corner of the enclosure. If this is the problem, you may need to try adding dessicant pouches to your enclosures. 

In either case, you may want to look at some kind of conformal coating to waterproof and protect the circuit board like this:

 

Read "Amazon.com" on

Amazon.com

Hope that helps.

Akiba

Hi Sam, 
I agree with @Freaklabs. This could well be condensation.
I had several of theses corrosions and I now apply spray on conformal coating on every 
board before first deployment. I also discussed this with the audiomoth makers and they 
say it´s fine to use. 

Greetings from the cold and wet forest,
Robin 

Hi Sam, 

Thanks for sharing your experience.
Yes, it looks like the acoustic vent sticker on the IPX7 case has been damaged.

How long were these deployed for? 

There will be two spare acoustic vents stickers in the box with each case. Before you deploy them, it is worth checking that the acoustic vent in the case is in good condition. To check this, hold an opened case to the light and inspect that there are no holes in the vent. Sometimes birds or insects will attack the membrane causing very small pin holes, but even these small holes can be seen when holding up to the light. Any light that passes through means the membrane is damaged and needs replacing. 

What orientation was the case? 

From the ingress mark in your picture, it appears the case might have been deployed upside down? When orientated upright, with the clasp on the top (see picture below), the microphone opening has a small rain hood for rain protection. The underside of the microphone opening is cut away to allow water to run away from the opening of the microphone. When deployed upright the design prevents water pooling around the opening. 

I hope this helps.

Thanks,

Andy