These occurrences can give you a quick cash boost, but as the name suggests they do appear completely randomly. If you go to the right area but they don’t show, try again at a different time of day or with another character—you’ll get them eventually!
These appear as blips on your radar at various locations, and to take advantage of them you need to blow the back doors off (sticky bombs work best for this) then grab the cash bags that drop out before evading your newly acquired wanted level. If you catch the guards loading the van up, you can take them out quickly to collect the money without attacking the vehicle, and each of these robberies should net you at least $5,000 a time.
On Little Bighorn Avenue in the Rancho district of Los Santos, you’ll see a red blip appear on your radar as you spot a cyclist being separated from their ride. Chase down the thief and eliminate them, then return the bike to its grateful owner. You’ll later receive an email, informing you that the citizen you helped out is actually the owner of Animal Ark (an in-game pet store chain) and he’s gifted you shares in his company for your actions. These can then be sold on the stock market to cash in a cool $100,000.
In North Paleto Bay you can find a random event, where two guys are in the process of burying a woman alive. Dispatch them both then collect the unfortunate victim in a vehicle and take her to the waypoint that appears in Vinewood Hills. On the way she reveals that she’s the daughter of a mob boss, and shortly after you deliver her you’ll receive a call to inform you that you’re now $60,000 richer for helping her out—nice.
Head to the end of this track on the east side of Mount Chiliad, where a marijuana leaf icon will appear on the map to reveal a Weed Farm you can attack and steal drug money from. Enter the area and eliminate the enemies guarding the farm, then check inside the barn to recover a case full of cash containing anything up to $100,000! You can also return every couple of in-game weeks to hit the farm again, making it a great source of extra income.