Hi Grizzly
The block wall could create a trip hazard in the event of a hasty exit. I presume it has been put there to stop any liquids escaping from the plantroom.
With regards to the new sign Explosive Atmosphere.
We have a new standard now and it is based on ISO 817 which I think you use in the UK and Europe.
Ammonia has been reclassified from a B2 to a B2L refrigerant and class 2L is now considered to be of lower flammability but not explosive, I think it would be the same there.
Regards
Paul