What kind of flat is this? I'm stumped. For the last yew years I have been getting flats that look like this:



As you can see, it's a tiny, tiny hole that can only be found by inflating the tire to twice its size or so. I know these aren't puncture flats because there is never anything in the tire, and they don't seem like pinch flats because:


They never come in pairs
They happen much more frequently on my front tire even though my rear wheel has much more weight from loaded 40L Ortlieb paniers.
I'm running between 110 - 125 PSI, and more pressure doesn't seem to reduce the rate of flats.
They tend to show up when I go to get on my bike in the morning, rather than on the way or after work. (Though this doesn't really contraindicate a pinch flat, I just thought it was weird).
They seem to mainly be on the outside, although sometimes they are on the inside of the tube.


Lately they've been happening more frequently, today I had three (I found my bike flat in the morning, then tried two more patched tubes with similar flats that I had missed). 

There are the usual gotcha's that I usually try to avoid too:


Watch out for lips/edges on the road.
Only use tire lever for tire removal, not replacement.
Put some air in the tube before putting on the wheel to avoid folds/bends
Make sure rim tape is fully covering all spoke holes


I'm getting tired (pun not intended) of these, so before I buy a new (bigger) wheelset, I thought I'd call for help.

Thanks!

Some more details: Mavic CXP 30 700c wheels with Continental Gatorskin 700x23c tires, 4 mile commute each way over decent roads in hot conditions.
That looks like the sort of hole you get from tiny bits of glass working their way through the tyre, then slowly cutting into the tube. I'd check the outside of the tyre for tiny cuts, and the inside for barely-detectable points poking through.

This photo shows the sort of small cuts I'm talking about:



Each of those had a little bit of glass or something in it once, but most of those work their way back out. The problem is that the ones that don't usually work their way in instead. If you poke in there with something like the file shown (a sharpened spoke is what many bike mechanics use) you'll sometimes find a ~2mm or smaller triangle of glass. 

When those work through the inside of the tyre they don't punch a hole like a nail does, they slowly cut a little slit as you see above.

The way to avoid this is by going over your tyres every few weeks and digging out those bits of glass.

Note that even very puncture-resistant tyres will suffer this, just less often. I've had Marathon Plus tyres puncture this way, but only once (I use those tyres a lot). This guy had the same problem with tyre liners. Picking the glass out is the only solution.

Echoing @DanielRHicks comment: one annoying possiblity is that you have a little fragment of glass or something loose inside the tyre. So every time you check the tyre you don't find it because it slides around as you rotate the tyre checking it, and every puncture is in a different place. So check for that too.