It was before the cops got there but I helped a security guard trying to apprehend a shop lifter who had run out of the mall, across the street and into a scrub brush area on his way to a housing area. I jumped into this at the point where the guy and the guard were entering the scrub... the guard was spent from chasing the guy through waist deep snow, so I called to him, told him I also worked security (at the time) and offered to help.
He told me the guy had a knife (Fun) and I took an angle on the guy until I was between him and the housing area. Lucky for me the guy was also tired and I could move faster through the snow than him. The bad thing was that a school bus was letting kids off about 75 yards away and I was seriously concerned that the guy might get stupid, so I held my ground. Luckily he only faked a charge at me and then changed direction parallel to the back of the houses along the highway
Long story short (too late), I was pacing the guy when the cops came on scene from the highway and started pointing guns and telling him to get down. I stopped REAL fast then. It was funny, but I stood around to make sure they didn't think I was fleeing, but they didn't even talk to me or anything. I walked slowly back to my car, in case they started calling or suspecting me, but nothing. I didn't ask if they wanted a statement or anything because I was applying for LEO jobs and didn't want my actions to be confused with an "audition" for them.
I have to assume that the security guard called in my description when it was reported to the cops...