What is the Army's policy on calling " Attention" or "At Ease" in the Maintenance Bays?

If we want to get technical it the same for any other room. When an officer/ NCO of a higher rank enters and or exits you give the appropriate courtesies. However in my experience this courtesy is waved off, because the motor pool or mechanics are so busy staying ahead of scheduled services , having to stop many times to call Attention or At Ease would slow down progress. However since it is a battalion motor pool it should only be the battalion commander or CSM which is not much to says for, they have earned the respect. However most will wave this as well. It is up to each and every command team.

That depends on who came in the area, an Officer or the CSM. I assume you are referring to an officer, if so it is attention. Those that can stand and those that would have issue standing just stop in place until carry on is given. There may also be local policies, but they have to come from the Battalion or higher. Most times when anyone above the BnCo will visit you will know in advance and it will be well coordinated.

I am no stranger to a maintenance bays and maintenance tents in the field since my first job in the military was supply which put me in the motor pool. Calling appropriate courtesies like the OP said, "attention or at ease" will pose a safety concerned. If the shop foreman is having a meeting with everyone in their maintenance cage or if everyone is in the parts room by all means, exercise courtesies to the appropriate service members. If SMs are engaged with work which require a high attention to detail, do not call "attention" or "at ease" for at anytime the over head crane might be in use, heavy truck tires are being changed, the parts clerk is dropping off parts, a mechanic is measuring oil he or she pumps from the bulk oil container, engine packs are being removed or installed, etc but giving the greeting of the day is okay.