I go climbing at a local gym at least a couple of times a week if I'm not climbing outside (which sadly, is pretty rare these days). There's always new faces in there, and of course regulars who show up every week on the same nights as myself.
I tend to keep my eye on folks I don't recognize or who look new to climbing, just to evaluate how they are belaying their climber. A "belayer" is the person in charge of holding the end of the rope with a belay device, which keeps the climber at the opposite end of the rope from hitting the ground should they fall. A belayer also lowers the climber when they are ready to come down.
I keep an eye out because over the years I've seen a couple of people dropped from bad belaying techniques, and I knew before the drop happened that something was up. And both times I've regretted not saying anything prior to the fall (even though nobody was seriously hurt in these instances). As a result, over the decades, I've developed a kind of sixth sense for feeling out when something might go wrong, someone needs a little adjustment in their technique, or even an encouraging word to just relax a little. It's been pretty useful a number of times.
So last night, I witnessed a belayer who kept releasing their brake hand while the climber was up on the wall. You don't ever let go of your break hand when belaying. If you do and the climber falls at the same moment, he or she will likely hit the ground at a less than optimal speed. Aka "decking". There are padded floors everywhere, but you can still get seriously hurt depending on how high up you are when you fall.
So I walked over to let the belayer know how to make things safer (and in a polite and depreciating way, which is usually much more receptive). But while doing so I look down at his belay setup and have a minor internal freak out. The belay device is setup all wrong. Very much all wrong in a very, very bad way.
I quickly tell the belayer "hey, this is all wrong and I'm taking over", set up my belay device past his and as I'm doing so call over to one of the gym employees to come quickly, take a look at this, and be a general witness in case the belayer decides to get angry or aggressive towards me.
The employee quickly realized what was happening, encouraged me to continue what I was doing, and the climber was safely lowered to the ground. The belayer and the employee had a chat, and I walked away since this part was none of my business.
There's more details I could go into, but that's not the point of this story. What happened next is.
I walk away to another part of the gym with my climbing partner, and let the employee take over with the bad belayer. But then people start walking up to me saying things like "wow, I'm really glad you said something, his belaying was super sketchy" or "I can't believe that guy was doing that, it was freaking me out a bit".
At least four other climbers saw the bad belaying, and did nothing.
There's a certain etiquette in a gyms towards other climbers. You don't give out unsolicited advice to climbers about how to get past a hard problem. You don't loudly talk about "how easy that climb was" when someone else is currently struggling on it. And in general, you let people belay how they are as long as it's safe, even if it's not your preferred technique. If someone is belaying with an ATC but you prefer to use a Grigri, you're not going to comment on it.
And I think it's this mini-culture that kept other people from speaking out, combined with maybe a bit of uncertainty or embarrassment.
So what should you do if you see someone being sketchy in a climbing gym?
The simplest and best solution is to mention it to a gym employee. The employee will then (hopefully) casually walk over and observe the situation, and make a decision based on what they see.
I've talked with numerous gym employees about this, and every one of them absolutely wants to know if someone is suspected of being unsafe in the gym. So don't feel embarrassed by this. Don't think you are lacking experience to make the right call. Just go let the employees know. You might be wrong- so what? It's better to be safe than sorry, especially when someone could potentially be seriously hurt.
I feel at this point, I need to defend gym climbing a little bit. In 20+ years of climbing*, I've only had to take over someone's belay maybe once or twice. What I did last night was super rare. Even having to comment or report on someone's belay style is pretty rare. Maybe 2-6 times a year?
Climbing indoors is safer today than it's ever been. The belay equipment is modern and amazing, harnesses are more comfortable than ever, and new climbing gyms are popping up all over the country. The collective wisdom from all of this has increased the safety of climbers 10x.
One of my favorite analogies is to compare climbing to driving a car. Of course it's dangerous! But climbing is no more dangerous than driving- in fact it's probably safer. You wouldn't just hand your keys over to a 16 year old who's never driven and say "here you go- be back by 10!". That would be insane.
Instead you take a climbing and belay course, or learn from someone who knows what they are doing. And just like you have to pass a test to get a driver's license, climbing gyms make you pass a test to be able to belay someone. (I'm not sure the person who was belaying last night had passed the test- I wasn't privy to that part of the conversation. I can say that I've seen 8 year olds belay better than that guy).