"Fine." Lan Zhan is not convinced that this is a good idea, but hey, free excuse to hang out with Wei Ying. 

"Awesome. Thanks!" she says brightly, and hangs up. 

Wei Ying does not tell her siblings about the kidnapping thing; they know about her new interest in monsters, but they don't have all the context that Lan Zhan does, and she doesn't want to unnecessarily worry them. 

Wei Ying and Lan Zhan don't find anything by physically searching the bills. Wei Ying "borrows" a geiger counter from their school's science lab and they fail to find anything that way. They try placing various radio receivers near the bag and twisting the dial to see if it picks up any kind of signal from it. 

This is probably not actually the state of the art in finding tracking devices but it's what they think of to try. 

The next time Lan Zhan goes to visit her brother at Stanford and Wei Ying tags along, she brings the bag, stuffed into her backpack, and keeps a sharp eye out for homeless magical girls. 

Homeless magical girls do not appear to be lurking near Stanford.

While Lan Zhan and her brother are having sibling time that Wei Ying shouldn't intrude upon, she explores the city, starting in the vicinity of that one Mexican place. 

With enough concerted looking she can locate a shabby magical girl depositing bottles into the machine at a grocery store.

Wei Ying waves. "Hi! Remember me?"

"- hi! Sorry, I'm not great at faces...?"

"I asked you a ton of questions and bought you a burrito."

"Oh, yes, I remember that. Did you think of more questions?"

"Sort of! I asked a bunch of questions online, and someone wanted to know where I got the information I already had, and I mentioned you in general terms, and," she pulls out a printout of her conversation with noon9834 and hands it to her. 

"...huh. I have no idea who that is."

"I made a sketch of her after she kidnapped me but if you're bad with faces it probably won't help. Did you ever meet someone with the power to control monsters? That's my best guess at what was going on with her but I could be wrong." 

"I've heard... rumors? I don't think I've met anyone with that power."

"Fair. Anyway, she kidnapped me, in my sleep, I woke up inside what I can only assume was a monster, I have a sketch of that too," she produces said sketch, "she continued to want to know where you were and assert that she didn't mean you any harm, I was EXTREMELY skeptical of this on account of the kidnapping, we bantered for a little while, she called you Jiejie and said she'd done it before, she inquired after your well-being, I told her that you seemed basically okay but plausibly resource-constrained, and she gave me fifty thousand bucks to give you and let me go." She digs out the bag of money and holds it out. "I can't swear that it doesn't have a tracking device on it but we couldn't find one."

"I am so sorry that happened to you and really wish I had any idea why it did! I'm sure lots of people have called me jiejie, I lived in China for hundreds of years and it's just what you call a girl who is older than you if you're friendly, or casual, or six years old." She flips through the stacks of bills, a bit disbelieving.

"I would have been less concerned by her acting casual and friendly towards you if it weren't for all the stalker tendencies and the kidnapping," she explains. "I'm fine, I didn't even get caught out of the house, Lan Zhan was more upset about it than me when I told her." 

"Your quiet friend?"

"Uh huh! She's my best friend, her brother goes to university nearby."

"Oh, you don't even live around here and you got threatened into couriering to me?"

"Well it's not like your stalker knew where you were! That's the whole point."

"Well, I'm not sure who wants to stalk me and give me thousands of dollars. I'm just going to try not to worry too much about that, I guess."

"Good luck. I guess being as old as you are gives you plenty of time to accumulate stalkers."

"I guess! I'd expect most people have forgotten about me more or less by now."

"You don't have to be especially interesting along any particular axis to attract a stalker, it can happen to anyone."

"I guess I don't know much about it."