This is not invisiblity. It's putting your safe door behind a curtain. Yes, fewer people are seeing the safe, but the curtain is not actually increasing your security, it's just preventing casual attackers from seeing you as a target of opportunity (who, for the most part, are also not going to get past real security, anyway).

I don't know that it's dangerous, but it's not really additional meaningful security, in my opinion, so long as it has zero downsides. The example of the NSA keeping their algorithm hidden does have downsides. One of the biggest downsides of doing some non-standard thing is that you might not realize that you are actually weakening your security. If you have some protocol that involves not revealing the existence of a server to casual attackers, that's completely fine with me, but it's worth noting how easily these things can get conflated together.