So my youngun has been using one of my stainless Nalgene brand bottles. She just showed me two pinholes where it has been leaking. Doesn't appear to be dropped or dented just holes. Strange.

They got a prankster at work?

A coworker once drilled a tiny hole in another guy's coffee travel mug, an inch down on the side he drank out of. It only leaked when he tipped it up and dribbled all over his front

I have a couple thin foam turkey decoys. Looked at one and was wondering what the holes are. Pretty sure its the same size as a bb from a red ryder. My son has a red ryder.
Probably not the same but made me think of that.

I'd be concerned what's in my tap water to eat a hole in stainless...

I have a Stanley SS container that has holes in it where the bottom was spot welded on. maybe yours was as well.

I think if someone did that to my 80s vintage daily drinker wide bottom (last one in existence) Tim Hortons classic travel mug I couldn't be responsible for my actions.

I'm a prankster and an Engineer so it could be either .
I believe what you're seeing is Microbiological Induced Corrosion (MIC), I have seen this phenomenon takes place in 304 & 316 stainless steel tanks. Under certain circumstances, I have seen stainless vessels corrode faster than carbon steel- hard to believe but impurities in the water over time or liquids with a higher PH will form electrons that attack the surface of the stainless and cause pitting. This is the reason you see zinc anodes in water heaters and on boat motors, the zinc acts as a sacrificial anode that comes under attack before your $500 stainless prop falls off. Light surface rust on a knife or stainless sink is another example, I'm not sure if there's special care required on stainless bottles other than rinsing but other stainless can be passivated with citrus, i.e.' wiping knives down with 1/2 of an orange. It's this or you have friends like me.

Chlorine is very corrosive to those common grades of SS. I would guess it’s 304 grade , chlorine will degrade it and cause “pin holes” . I’ve made a living for 30 years dealing with it.

Cause of pinholes
I have seen stainless contaminated and form rust by using a steel rotary brush on it or a contaminated grinding wheel.

I have seen this several times in my life long welding career. Sir, you give a very good explaination of what that cause is. Very close to the same thing as what happens to steel pipe in gas lines, only the cause is different. It might be hard to attatch an anode to the cup mentioned! Thanks for the good explaination!

Back when well build lumber dip tanks ,we would see these sometimes. sometimes the metal would blister and come apart, metal company always replaced metals for use , finally said they found the culprit to be in recycled metals contaminating at manufacturing. so i guess who knows.

call Nalgene and see what customer service will do.

sometimes SS has bad spots , I have a Chicago knife that has a hole through the blade in the middle seems to have been some impurity in the metal that rusted out or corroded out