The weedfarm/Drug Shootout is identified as a random event on a few online sources. However, it definitely doesn't have a red/blue dot to denote it's position, but a black roundel with leaf motif, like a store/safehouse/property etc. It also definitely says you should come back in a week to check on it.
It's definitely a random event as it's shown as one on the checklist map on Rockstar's site. You're spot on about the message to come back in a week, though. I'm not sure where the error lies as I don't know for sure if the event was repeatable before any patches.