You have valid doubt, so, let me tell you clearly how it happened (my neighbor who is “Major in Saudi Army” told me this).
First, most of 172 were just hidden trained unarmed persons spread among different districts where they meet in small groups in different cities (especially in coffee shops and gyms).
When the Saudi intelligence system discovered their plan (I don’t know how), they didn’t arrest anybody but rather prepared a 1 time plan to arrest them all at same TIME.
Thousands of soldiers participated in such a plan where the majority of 172 were arrested during the first 2 hours without giving them any chance to warn the others.
As for weapons and money, all were buried in desert and not with suspects. The small guns that were found in few houses were nothing compared to what was in desert.