Yes, but common sense does prevail.
If you did a "what if" for every process step, you would end up with hundreds of Preventive Controls. That's not the intent.
You have to do a HA of your process, which may include things like receiving procedures, etc. Is that "technically" a PRP, not really. It's just how you do business.
This is why risk assessment is very important. In your mixing process "could" there be metal to metal contact if the seals on your beater bars failed? Yes. But given the design of the equipment and historical evidence of that happening, what is the actual likelihood of it occurring?
Hazard Analysis and Risk-Based Preventive Controls is what the operative term is.
Metal Detection would probably be a Process Preventive Control, because you would say "possible metal inclusion from previous process steps" as your justification for the PC.
You would not create a PC at the mixing step for metal inclusion, because the likelihood of it happening there would probably be very remote.
Since you are a bakery operation, the biological hazard at the receiving step would be salmonella, e. Coli, etc, because there is historical evidence of contamination of flour (or other materials) with those pathogens.
The PC is not applied at receiving. It's applied at the baking step, where time and temperature will kill those pathogens. Of course you have to scientifically justify that.