The "deep undercover" idea intrigues me...
I'm not a LEO either, but I agree with the rest; bad guys- to a man- won't use a holster, for the reasons already given. (To tough to ditch, plus the fact that most thugs aren't the gun-nut type, and aren't about to wander into a Wal-Mart and pick up a holster for the price of a dime bag of Rock, holsters add bulk and make it harder to hide, etc.)
So if you're undercover and using a holster, the bad guy is gonna *know* you're a cop. And that's a good way to ruin a whole day...
Okay then, why not take some denim- assuming jeans like 80% of the world- and make a short strip of "second waistband" inside the regular waistband, and shaped like a belt-slide, "Yaqui slide", etc...?
Sew the denim with a rolled seam edge, and keep it rough-side-in, use thread the same color and style as the rest of the pants, and you'll have a "holster" that's essentially invisible when empty. If done carefully enough- heck, stick on the size & brand laundry tag for camoflage- it could even go unnoticed if they make you take your pants off to check for "wires".
More camoflage- sew the "inner waistband" all the way around, fly-to-fly. Then, if the BGs do make you take off your pants, or otherwise make a thorough search, it'll just look like a split seam- have a couple of loose threads strategically placed- and might even go less noticed than a 'string holster'.
The ultimate "Inside-the-waistband" holster...
If anyone tries it, let us know.