Father took her to pick up the rental (hmm, you mean you CAN rent a car in Yreka? Novel idea!) where she told him she would be gone for a few days and couldn't tell him where (Flores interview with Arias Father). The warrant was signed and she was under observation the prior evening to see if she would leave. Officers saw someone who they think may have been Mom Arias come to grandparents house that evening, talk outside to CMja, hug and then leave. During this time, CMja is loading stuff in vehicle.
Next morning, CMja goes to vehicle at which time she is arrested. Warrant served on Grandparent's home and parent's home simultaneously. Several knives inside books in boxes were found, along with 9mm ammo. NO GUN at that time.
Mom Arias later called LE to see if they had found 9mm. LE contacted rental place/or vice versa and it was found duct taped(?) to the engine block (?) although I think the firewall would have been more logical.
I don't know about the Calif/Mexico border, but the Az/Mex border has imaging technology suspected vehicles are run through to find anomalies in a vehicle. Guns are easily spotted. I doubt she was headed to Mexico because US rental cars are subject to search far more often than not, and specifically for guns and cash..........but I don't think she was that smart.