High CO2 concentration in frozen warehouse filled with cranberries

I work for a cold storage facility.  We have a freezer room that was recently filled with cranberry totes, that are in the processes of being froze out. We use anhydrous ammonia for cooling, we have only electric forklifts, no dry ice is stored in that room.  The room was filled with 20,000 totes over a 6 week period, during which the CO2 level did not change. Now, 2 weeks later the CO2 reading in the room has risen to 40,000 ppm. What would be causing the CO2 level to rise?  

There is a theory that it has to do with the gases coming off the cranberries.  However, our facility has done the exact same loading of the room with cranberries every harvest for the past 7 years and never had this issue before.  We have warehousemen that have worked cranberry harvests for 30 years that have never seen this problem.  And given that cranberries give off methane when they decompose, I do not believe the berries are the source of the issue.  I am at a loss as to what is causing the CO2 levels to rise as much as they have.  Can anyone explain this?

Have you checked your unit or CO2 meter. Or verified the number is correct. 

Did the company monitoring your units do anything recently? 

We used 2 different CO2 meters and got the same reading on both.

We do all maintenance and monitor in-house. The only maintenance done in the room was to the evaporators, which was ice removal and preventative checks on valves.  But again, we have no dry ice in the room and we do not use CO2 for cooling, so if there was a problem with the evaporators we would have high ammonia levels, not CO2.

Have you verified that the cranberries aren't somehow fermenting?  It is unlikely at freezer temps, but stranger things have happened. 

Also, it sounds like you don't, but if you do have any Co2 piping, perhaps there is a leak somewhere?

There is no CO2 piping, so there is nothing that can leak.

The room is currently at 1 degree Fahrenheit.  It's possible (but unlikely at this temp) that the Cranberries are fermenting.  However, cranberries give off ethaline when fermenting, so even if they are I don't see how they could be the source of CO2.

We've vented the room for an hour a day over several days.  The CO2 level has gone down over this time and is not building back up.

Chalk it up to one of life's mysteries. 

I have no answer.   But thats a little scary.   I don't think a person would survive long at, 40,000 ppm.    

I'm not familiar with CO2 meters, but are false positive readings possible? As in, some other gas in the air could trigger the meter other than CO2? I wonder if the manufacturers of the meters would have any insight into your issue, especially if these meters are used for detecting leaks in freezers.