I would suggest not lying (anonymous complaint “oh, I was just walking my dog…” style). Evaluate how you feel about it. If it was a big deal, don’t pass it off as “well, it was strange and I thought I should let you know about it…” If you felt threatened, say that. If you feel like it wasn’t a big deal (and I don’t think you do feel that way) then leave it alone. But you have to pick one.
Management should never reveal the source of a complaint, especially when the complaint is about aggressive behavior. A lot of times it’s pretty obvious where the complaint came from (because, really, most people know when they’re being douchebags) so lying about being an uninterested party, i.e. suggesting the complaint is from somebody who doesn’t even live in the building, isn’t going to save you anyway. However, there’s a very good chance Anonymous Passerby’s single complaint will not be taken seriously. If it were me, I would think that was very much Not My Problem and do absolutely nothing about it at all.
Don’t call the manager. Well, call if you want to, but don’t JUST call; provide your detailed complaint in writing. If you don’t, you’ll probably be asked to anyway.
Finally, if you feel like your complaint isn’t being addressed to your liking/fast enough for you, please don’t immediately start bitching at the manager. No matter how much we really want to help you, sometimes we have our own impossible hoops to jump through.
P.S. The cops don’t give a shit. Maybe if you’d called to report the suspicious behavior at the time, but there’s nothing they can really do about it now.