Missile-mounted cameras
Python 2, 74 bytes



lambda l:max(reduce(lambda i,x:i+(a+i*(b-a)==x),l,0)for a in l for b in l)


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Tries every possible sequence of the form a,b,... with a and b in the input l.

To find the smallest index i of this arithmetic progression that's outside l, we do something a bit unusual. Instead of counting up the progression to find an element not in l, we iterate over l, taking advantage of it being sorted. For each successive element x of l, we move to the next index in the arithmetic progression if its current value matches x, and stay at the current index otherwise.