No pocket holster will 100% guarantee retention. In fact, that's the one thing you can trust, nothing man made is 100% reliable and trustworthy. Not the gun, holster, ammunition, nothing. All you can really rely on is that none of it can be.
It's no different than pocket knives. Get upside down, they fall out. Hence the huge shift to pocket clips on knives. While I've bent a few clips over the years, the only knives I've lost in the course of a days work have been pocket carried knives. Never a clipped knife.
Cell phones? Same. Pocket change, yes. In fact, pretty much anything in my pocket will fall out, whether laying in the street hooking up chains to tow a car, or remodeling under the sink, or under a car in the garage.
Frankly, it's really a no brainer. Anything loose in pants pockets falls out when you get upside down. Precisely why duty uniforms - like in the military - have flaps and fasteners. To keep things in when you absolutely need to. Never lost anything out of a pocket in 22 years in the Army Reserves.
Sorry for your situation, sometimes we learn these things inexpensively, and here's hoping that's how it turns out. It's cost me a pocket knife from my mother in law, about 3.58 in change (who really knows,) and fortunately, not a Strider SnG. Turns out I put it down instead of clipping it in my pocket. That took three weeks to sort out.
You did the right thing calling. If it wasn't for the precise danger of them finding it, the boys would have been the best at it.
