Are undetected pen tests common for MSPs?

I am wonder how common it is for a MSP to NOT detect a pen test that is going on with an outside 3rd party. Maybe I am thinking of too harshly, but pen testers don't wave a white flag while they are testing, they use the same tools as hackers, and if they are not detected from pen testers you hire, how can I comfortably expect them to detect them from actual hackers? Would love your purview on this!

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