I worked for a private security & investigation service while I was in college. After the owner got to know me, I was often pulled off uniformed job sites to cover when he was behind in "cases". I'll tell you right now, most all are cheating spouse / insurance fraud / employee theft cases. Observe and document. Boring.
My best "case" was a manufacturing company that was experiencing breaking in / theft from the cafeteria ice cream machine. A handful of employees, and a large cleaning crew were there at night. The client wanted me to pretend I was a new man on the cleaning crew. I didn't think that was going to fly, so I scoped out the cafeteria. The only place I could figure to hide was a big, glass front walk in freezer, with mirrored glass. This was before cheap security cameras were common. Late 70's, early 80's.
Borrowed a heavy coat someone had laying around - It was summer - Then locked myself in the freezer with a stool to sit on. Had a great view of the cafeteria.
Two male cleaners came in, jimmied the ice cream cooler open, and grabbed a handful. Wrote down their descriptions. Over the next hour, maybe 7 or 8 others came in and all headed to the ice cream cooler. Then a small group came in, jimmed open another fridge, and started cooking food on one of the stoves.
I got caught when a young hispanic girl came up to the freezer, pulled the door (locked), then she peered into the freezer. She seen me, made eye contact, then screamed. Busted.
I left the freezer, went back to the security office, and contacted the owner. He came in, and called the local PD. He had the cleaning crew line up, and me identify the culprits. After about 8 or 9, the local cops said they were not arresting that many people over stolen ice cream. The business owner then fired the entire cleaning crew.
And that was my "Big Case".
Larry