It's interesting you're modelling the pairs as being unordered.
If person A's face unlocks person B's phone, does the reverse hold? I'd guess it's moderately likely, but not certain (e.g. the chance in the other direction becomes 1 in 1000 instead of the magical 1 in a million).
If person A's face unlocks person B's phone, does the reverse hold? I'd guess it's moderately likely, but not certain (e.g. the chance in the other direction becomes 1 in 1000 instead of the magical 1 in a million).