Snails probably came in with the plants. That's where ours came from

we have a lovely colony of snails .......too many grrr.
Ahem stop rant
Hillstreams like to hide! (different species hide more than others) Our Cheni are not that enthusiastic over blood worm or
brine shrimp but I know other people on here have Cheni that love them!. We have fed ours over the years:
Tetra Tabimin (green sinking algae pellets) - green
Nutrafin Sinking complete food tablets - pink - they liked these
Nutrafin Spirulina Algae tablets
Interpet Liquifry No.3 Baby Plus - very small granules mainly for fry
Interpet Liquifry No. 1 for egg laying fish
Frozen bloodworm, daphnia and shrimp (never too successful)
Blanched courgette and chard
(not all at once I hasten to add!)
They will also chow down on goldfish flake food if they float to the bottom
Currently they are loving the wonder food that Madduff recommended from the guy who makes his own on ebay.
When we were worried about our first Wui eating we would put some of the liquifry in the tank then we were sure he was getting something! Eventually he realised that the sinking tablets were food and would go after them but this took a very long time. He now looks sleek and well fed on the add time we see him!
I wouldn't recommend going looking for them unless you haven't seen them for a long time and are V. worried. This may just stress them. If you have rocks in the tank they are probably suckered to the underside of the rocks!
We have some Sinogastromyzon Wui in Our tank and you are very lucky to see any of them at all!
Good Luck!