... are the only enemy in Destiny that has one or more instances where they DIDN'T attack you on sight. The two occasions are in Paradox and Blood in the Garden, both are coincidentally key parts of the No Time to Explain Quest line.
The first is easy to explain, they can't beat the Taken alone.
But the second...
Isn't it odd that a race of killing machines that follows a strict pattern would ALLOW a Guardian to walk freely among the Black Garden, their supposed home*? It isn't until you shoot one that they start attacking.
*then again, how else was I supposed to get this nice pic?
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Silly guardian, believing that we make mistakes, we don't make mistakes everything is according to plan