New to the forum but thought I'd post on this topic first.
I'm a 15 year service LEO. Most of that, including current assignment, has been plane clothes narc or uc work. Most often I carry without a holster inside the pants. I have used rubber bands as mentioned or tire tubes to keep the gun from slipping down my pants.
The requirements of this type of work are far different than most. You may have a great $150 plus holster but if you back me up on a deal, you look like a cop and I can't have that. It's not that the bad guys don't want you to have guns, they expect them. It's just they want them to be guns they see alot on the street carried the way they see them. A nice holster is not that. Neither are those nice cop belts we often use to secure that holster. But that's another topic.
When I'm in the office on paperwork I often find that my chair or leg position cause problems with most holsters. Easier to slide the pistol around for comfort. Readjust when you get up to leave.
I carry a Glock 30, Glock 22, Kahr pm 45, and a J frame smith this way. Don't get me wrong, a holster is nice and probably the safest and only way some people should go. But for those with the experience, eye for safety, and necessity dictated by the job; no holster is a viable option.
Anyway, that's my little 2 pennies worth of input. Nice to be hear.