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Yeah, they like a high humidity so no doubt they are under something, of course they might have been eaten but the first thing they would have done is found something to hide under. Even in with frogs and lizards I've found a certain amount are living under rocks and wood and maintaining a small population. I've found that for a lot of animals they are not one of their favourite foods, most will only eat them as a last resort. I don't have experience keeping them with tarantulas though, but scorpions don't tend to bother with them.
But still, you only added a few so it may take more to establish a permanant population, they could have been picked off one by one before realising that they were being hunted.
They have this weird thing where they are chemically sedated when in physical contact with others so they tend to run around looking for a group to hang with if they think they're alone. Once they find others they stay together under cover most of the time, especially in a drier environment. I'd still add more if you can without going to great lengths.