The pins (or the other T/N and AD listed devices) can be put in place if the coupling is fully connected... or completely unconnected. On my 15 I can see the four L'Hotelliers quite well but I still first check that they're assembled properly by using the pull test and then carefully visually checking the connections. I then follow those up with a positive control check.

I have seen people get the couplings sort of "half-way connected": the wedge gets pushed down, the socket is put part way down the ball and then the wedge is released. In this situation the pins won't go in (or the Wedekinds won't slide home) so the few times I've seen it done it was always caught quickly.

One way I've theorized that the L'Hotelliers may have disconnected when not equipped with the safety devices would be if the spring loaded wedge got contaminated with enough dirt that the spring wasn't able to push it back up to lock the coupling when it was released after putting it over the ball.