Hi Droptine
Not sure how old your plant is but I have experienced leaks under insulation due to corrosion which only became apparent when the plant tripped and the pressure increased.
These are obviously very hard to find but if you look around to see where the insulation has had the surface damaged and there is ice build up it is generally a good place to start.
As Josip has said you have no choice then to isolate parts of the system and allow the pressure to rise.
If this is a freezer plant you should be able to shut it down for a while after hours and allow the pressure to rise and use sulphur sticks as stated by Josip. The plant could be off for several hours without any loss of product temperature.
The problem is if you don't do anything about the leak you are not only sucking air in but moisture as well and this will cause other problems as well as make your plant less efficient due to ammonia's affinity with water.

Hi Magoo, I prefer the term experienced person rather then old person lol.

Regards
Paul