Not just a while. The US Patriot Act gave powers to allow border controls to hold people indefinitely without cause.
The US is a massive country that ~75 million foreigners visit each year. In 2018 that figure hit an all-time record high near 78 million, which is in curious contrast to your setup premise.
With so many people visiting the US every year, you surely can produce a very large number of examples of toursts being held indefinitely - for many years even, one imagines - without cause.
Take the anecdote of the person to whom you replied. Lots of people will tell that person's friend, "oh, just let it go, the border agent didn't really mean anything." And if the person complains to a higher authority, the complaint is staggeringly likely to get brushed off, if not used in exactly the opposite way the person intended when filing it.
At a previous job of mine, I worked for the phone-based customer service while another team in the same group operated service windows for in-person assistance. No authority here at all, just answering questions. More often than not, if someone filed a complaint about how one of our window clerks had treated them, the supervisor delivering the complaint to the clerk treated it as a joke. "Well, it says here you told the woman she should smile more. I pulled the security tape and she was pretty cute. Sure would have looked nicer if she had smiled, yeah?" Yeah boss, sure would have! "Darn right, can't fault you there, so we'll just toss this one as unwarranted, yeah?"
Holding people indefinitely without need of cause, and without a clear legal way for them to communicate with other people, there can be no information on this. It's like asking for pictures of the inside of a black hole, or a message when you reach the area that causes your phone to explode. And on the way out? They can force them to sign anything to say that it didn't happen.
Yes, this does sound very conspiracy-esque. But we're talking about a country that tried to assassinate Castro 600 times, and several times poison him to make his beard fall out. Doesn't that also sound crazy? We have clear evidence that the US has these powers, and thanks to guantanamo and what's currently happening in the ICE camps, we have clear evidence that they will use that sort of legislation when it suits them. Even if it clearly violates international and domestic human rights laws.
Another answer is that the sheer sum of people that that is happening to, doesn't matter. It's the fact that the government has granted itself those powers in the first place, that matters here.
As an analogy, I don't care that nobody in the camp has hit my child over the head with a large stick, I care that they have gone to the pain of stating, in their code of conduct and in the contract of attending the camp, that they can hit my child over the head with a large stick if they wish, with no repercussions, and that my child is responsible. Do you see?