Apparently, Pieter Wuille (sipa) is signing commits with RSA key ID F8F0B657, the public key of which is not listed anywhere meaningful (if anywhere at all).
In particular, at this location:
there is listed (among other developers' PGP keys) Pieter Wuille's key with ID 1DAAC974, which is also mirrored by MIT's PGP keyserver. However, this is not what he's using to make the signatures for objects in Bitcoin's git repository.
