The title sounds like this is the most useless page ever. It could be. Yet, I wish someone had made a page like this available in 2004, when I started exploring Thailand in search of Nepenthes. If you know that someone has been looking for Nepenthes in a place and he couldn't find them, you can skip that place, or you can change methods, or you can check areas of the place where he hasn't been. Otherwise, you'll just keep making the same mistakes. The only caveat here is that there's maybe one or two non-Thai guys every century who will travel around Thailand looking for Nepenthes, so I don't see any crowd watching this. Meanwhile, I'll enjoy the trip down memory lane.
The locations where I did find Nepenthes are not being ignored. They were treated in my book Nepenthes della Thailandia (2010), in Italian. They will be treated in English in a book to be published in fifty-three years.
The ones listed below are not all the places where I didn't find Nepenthes. These are reports from an ancient era, which would be a shame to waste. Later "non-locations" are just GPS coordinates followed by "nope". Here I'll also complement my old material with present-day data and photos.
You will often read about me asking the locals about pitcher plants, and you might think that probably they don't know what I'm talking about and that's the reason for my many setbacks. In loco, I used the Thai name for pitcher plant, which is a funny oddity known by everybody. Here I use "pitcher plant" for simplicity. A handful of times, I've seen people confusing Utricularia, Drosera, Paphiopedilum and orchid pseudobulbs with pitcher plants, so that could be where some setbacks come from.

Trip 2004: Mt Phanom Thong Mt Hin Rong Kla Chat Trakan Falls Na Haeo Mt Ruea and Mt Luang Kaeng Chet Kwae Sak Yai and Khlong Tron Ramkhamhaeng Lam Nam Nan

Trip 2006: Mae Wong Mae Wa Chiang Dao Huay Nam Dang Mt Inthanon Sri Nan Mt Wiang Mt Phan and Mt Pha Yon Mukdahan Kaeng Tana Thale Ban Si Khit Falls and Mt Nan

Trip 2007: Mt Yai Ngao Falls Tai Rom Yen Khlong Kaeo Falls Trip 2008: Libong Isl Sukon Isl