The fact that S(P2) mod p doesn't tell you anything about whether 2^P  1 may be prime when P = 2^p  1 is prime, disproves the existence of a general "S(p2) mod p" test for Mersenne primes.
More important than this specific case IMO is the fact that S(p2) == trace(u^e) (mod p), where u = Mod(x+2,x^2  3) and e = 2^(p2)%m, where
m = p  1 if p == 1 or 11 (mod 12)
m = p + 1 if p == 5 or 7 (mod 12).
We know that u^m == 1 (mod p) so the exact multiplicative order of u (mod p) is a divisor of m. I have not investigated when it might be known to be a proper divisor.
If there were some pattern in S(p2) (mod p) that indicated whether 2^p  1 was prime, presumably it would somehow be manifest in the "reduced" exponent e = 2^(p2)%m. I am not aware of any convenient formula for this remainder, but I don't know that there isn't one.
So far the "trivial" pattern when p is a Mersenne prime is the only consistent one I am aware of, and in that case S(p2) mod p utterly fails to indicate whether 2^p  1 is prime.
