Well. Considering I have no idea what happened after Monika became a Magical Girl, her biography kind of ends there...
1. If someone starts out with an attack on a Witch, or maybe starts out just having eliminated a Witch, I'd possibly place Monika just randomly somewhere in the vicinity to find.
2. If nobody attacks or destroys a Witch, I could envision that she would have fled from the Witch. After that, she'd find and stalk other Magical Girls, only revealing herself after confirming that "Yes, this is a person that could potentially treat me nicely"... or until she's discovered to be stalking said Magical Girl, in which case she either flees or stumbles. Haha.
Edit: ... Or that could already have happened and I simply join up with someone else at the start. Eh, that's a possibility. Otherwise, if she didn't find another Magical Girl for long enough, Monika would eventually be forced to hunt a Witch herself. So, well, I got multiple options.