On nfs3 we've had
rpc.mountd which logged nice messages to syslog like
authenticated mount request from 192.168.1.11:839 for /mnt/music (/mnt/music)
nfs4 is quiet silent by default. I wish I could log authenticated and more important unauthenticated mount requests (including client IPs)
I've played around with
rpcdebug, trying all kind of modules and flags but usually it logged too much and client IPs were not shown.