In most cases saltwater should never be added to the system without a specific reason. Replace water removed by skimmer, a possible leak or spill, or when doing water changes. And it should be checked with a salinity device of some sort. Assuming you have no way to check the salinity and If none of these conditions above has occurred, it is safer to add RO/DI water.Return chamber does look dry
By the looks of it he has an apex system so he should notified of the problem from the app
But right now if just added a thew litres of sw or just enough to cover the return pump it will solve this problem for short period
My guess is may be auto top off empty or playing up
Maggifishh is Sounds like you handled it well!
Welcome to R2R!