Maximum Alienness: What might make life hard to recognize...?

Maximum Alienness: What might make life hard to recognize as life?
By Caleb A. Scharf 
https://blogs.scientificamerican.com/lif...alienness/

Even though we still struggle with finding a satisfactory definition of life, that doesn’t mean that we can’t think about ways that life might be so different, so alien, that we would also struggle with noticing its existence. 
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For example, considering life so different (or advanced) that its fundamental substrate is radically unlike ours. ... if dark matter does exist it must also constitute the majority of matter in the universe and therefore represents an awful lot of juicy real estate.
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Similarly, what if complex, thinking biological life is fleeting on the cosmic scale but its machine progeny are more robust and more widespread? Such entities might also be very hard for us to recognize as such, either whizzing around at high velocities between the stars or massively encrypted in their fundamental design and their communication strategies.
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I suspect that one characteristic might be to do with our perception of the passage of time. We are biased to be sensitive to a relatively narrow range of timescales for events.
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On longer timescales I think the possibilities are many. Complex interactions that form a living system could take place over thousands, millions, billions, or trillions of years. Interactions of matter that are extraordinarily subtle by our standards might play out over very, very, long periods.