Does 怪しまれず take the に in 誰にも or has the second に after も been omitted because it can not be used after も?
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(He) was able to move around without being suspected by anyone.
This is 誰も (anyone) + に. So the に is the 'by' part used in the passive voice, as you suspected. There is no need for an additional に.
So why is it だれにも rather than だれもに? It turns out that the particle も likes to move to the right. I once asked a similar question here .