There's some confusion about what "Tanken Nichijou no Machi" is.
It is not the thing you've seen subtitles for. The things you've seen subtitled are a promotional video and some unaired next-episode previews, usually the 3 episodes mislabeled as such. Typically, these things are not construed as "specials", but in this case, they've been incorrectly conflated with and mistaken for "Tanken Nichijou no Machi", which MAL thinks are specials.
"Tanken Nichijou no Machi" does not consist of a promotional video and some unaired next-episode previews. Rather, it is a series of ~10-minute live-action clips featuring the Nichijou cast and staff (mostly Shiraishi Minoru, really) doing things that
have hardly anything to do with the actual show. Here is the first of them on YouTube (could go down at any time): https://www.youtube.com/watch?v=MXdhyyWeX8Y&list=PL0_XyQ09RMgHEGRTBch8uhqsrYRVfBedK .
They're kind of interesting to watch, I guess, but again, they have absolutely nothing to do with Nichijou besides featuring the cast and staff from the show.
(I suspect that MAL is the ultimate source of this misinformation. If somebody knows how to get an entry delisted from MAL, they should probably do that with "Tanken Nichijou no Machi")