Read the strings listed in that post and paste them one by one into the CTRL + SHIFT + F menu.
Make sure to have hidden services on in the explorer aswell as these promptpurchase scripts tend to be hidden in areas such as InsertService. (insertservice is hidden by default)
Thanks everyone! After many hours of looking for the problem it turns out that one of my plugins was causing the problem (DO NOT INSTALL: https://www.roblox.com/library/4898923432/Load-Character-Pro). I quickly installed this one thinking it was the original.
The reason why it was so hard to find was because it happened to insert a script into a model of mine that the game inserts at a random moment during the game (and only the main game). So even once I uninstalled the plugin, the model would bring the script back.
On a side note, this seems to be a big weakness for plugins in ROBLOX. Iām not quite sure what can be done, but a security update for plugins would be much needed. Of course being very cautious with which plugins you install is probably the best protection there is.