In industrial environments -— especially in automobile industry - "Mahlzeit"— “Mahlzeit” often simply means: "hello" “hello”.
Even most Germans who are not working in such companies do not know about this.
When starting a new job in such a company they are a bit confused because they understand: "I“I wish you a tasteful lunch"lunch” instead of: "hello"“hello”.
The response to the greeting "Mahlzeit"Mahlzeit differs from company to company.
In companies where "Mahlzeit"Mahlzeit means "hello"hello you can simply respond with "Mahlzeit"Mahlzeit, too.
In environments where "Mahlzeit"Mahlzeit means "I“I wish you a tasteful lunch"lunch” you simply respond with "Danke"“Danke”.
In a company I have worked for the response was sometimes:
"Mahl' deine Zeit selber"
Mahl’ deine Zeit selber
(Because "Mahl' Zeit"“Mahl’ Zeit” would mean: "grind time" “grind time”, so the response is: "grind “grind your time yourself"yourself”).
However you should never (!!) responserespond this way unless you are absolutely sure that this is the correct way to responserespond in the company you are working at!!