Prober, just caught this at a late stage & I have not read through all of the previous. Apologies now for re-stating what might have already been discussed. I too flew 737,757 & 767. Pressurisation systems were similar if not identical. AUTO is a setting which permits fully automatic control of the dump valve. It is the normal ops setting. MANUAL was really meant for use if the auto system had failed. Manual gives electrically AC or DC control of the valve but through pilot inputs on the CLB/DEC rotary control. You are absolutely correct in stating that flight at any point in MANUAL would be easily detected by human sensing. The ears take a real wallop with insensitive use of the CLB/DEC control in MANUAL. Take off in the MANUAL position would have been instantly recognised as the cabin would climb / descend (whatever) at the same rate as the AIRCRAFT rate.
If the crew took off in MANUAL, they will have noticed immediately and switched to AUTO or at least manipulated the CLB/DEC facility in order to stabalise the situation in MANUAL & then go to AUTO. If, however, all was normal & the flight was conducted in AUTO, a failure, sometimes subtle, would in all probabilty still be "sensed" by the pilots and switching to MANUAL in order to attempt control of the dump valve would be a standard 'non -normal' procedure.
Again, deep apologies if this has all been discussed previously.