Satipathanna Sutta and contemplating mind externally? More specifically, Buddha informs the Kurus thus:

"Contemplating mind internally. Contemplating mind externally. Contemplating mind both internally and externally"

How is one to contemplate the mind externally?

Additionally, in chapter 1 of the Shurangama sutra, the Buddha skillfully guides Ananda into locating his own mind by searching not just within the body but outside of the body. This throws Ananda and the other monks into mental and emotional upheaval (quite fascinating actually).

So, from the perspective of satipathanna sutta, is one to contemplate mind beyond the boundaries of the body?
Abhidhamma Vibhaṅga Khandhavibhaṅga:

Therein what is internal consciousness? That consciousness which, for this or that being, is personal, self-referable, one’s own, individual and is grasped (by craving and false view), (i.e.) eye consciousness, ear consciousness, nose consciousness, tongue consciousness, body consciousness, mind consciousness. This is called internal consciousness.
Therein what is external consciousness? That consciousness which, for this or that other being, for other persons is personal, self-referable, one’s own, individual and is grasped, (i.e.) eye consciousness, ear consciousness, nose consciousness, tongue consciousness, body consciousness, mind consciousness. This is called external consciousness.