I've had one VERY VERY close one in a little over 100 hours. Were it not for the mark 1 eyeball and quick reactions on the part of the FI sitting in the RHS I would not be here today.
That was SE England, not far from Pulborough, at about 2500 feet, >10K vis. about 3 years ago.
As IO540 says, the ones you don't see are the ones that will hit you!
The FI (not me) estimated that we missed by less than 30 feet - and that was AFTER taking last-second avoiding action (we dived, just as well the other aircraft didn't).
SD