1) I'm pretty sure that Mission Trails Regional Park is under the jurisdiction of the San Diego Police Department, not the Sheriff's Department. Also, I don't think that mentioning it in casual conversation counts as "reporting" it. That being said, your info is pretty vague.
2) as you probably know, Mission Trails has undergone HUGE changes since the 1980s/90s. There is now a nice bridge across the river (I recall walking over a pipe in 1986), the trails are labeled and much more extensive (late 1990s), the road thru the park was converted to one-way+ a bike lane, the 52 freeway was constructed and built (opened around 1994 I think), the visitor center was constructed and built (also opened mid-late 1990s) and much of the area burnt in the 2003 wildfires. My HS cross country team used to do workouts in Mission Trails in the early 1990s. There was an old 1940s-era car down in Oak Canyon, but that's gone now. We had SO much fun exploring as we didn't have access to any sort of map, but now everything is clearly spelled out and there are 10x as many people.
HOPEFULLY a body would have been found but you never know.
3) The area just north of the park (north of the current 52 freeway) seems to be under some sort of military jurisdiction (Eastern edge of Miramar MCAS). I have hiked in there, but you can read the internet and find stories of mountain bikers in that area who have been "escorted out" by military personnel. You may have heard some sort of legitimate target practice from a few miles away.
3) I haven't personally heard of/seen drug activity in the area. It's really known more for hiking and mountain biking.
4) Do you remember what day of the week it was? (ie weekday versus weekend? Maybe you would remember if you were ditching school, or if you would never have done such a thing?)
5) I am aware of a missing person case involving a ~30 year old woman from spring 1986, San Diego. Her story was all over the news at the time. Unfortunately, she is not in any missing person databases, even though she was never found. I have long considered posting her story here but the only internet remnants of the case are on the San Diego Union Tribune archives (late 1986 with updates in the 1990s saying that she was still missing), with access by subscription only. I'm not sure how to go about posting her story. If i created a thread for her, can I paste that entire newspaper story into the thread so that others may read it?