Man, some of you people are just ridiculous. The OP presented a perfectly reasonable story yet you still have to try and tear it apart thinking he's up to something, or just making it up. Why? What would be the point? It's not even an interesting story.
Some lawyers have no clue what goes on outside their direct sphere of influence. You take a corporate lawyer that sits around all day reading legal documents, he has no clue how a PI operates. This isn't Hollywood. And how do lawyers talk? They're typical articulate, with good spelling and grammar. Sounds like the OP to me.
Any of you who advised him to carry a weapon, or brandish a weapon, shame on you. Give your head a shake.
Any of you who told him to call the police... well, it's not a bad idea, but there is very little that will come of it (and yes there is no point for him to call while he's out of town). No laws have been broken. At the very best they will come and take a report. But they won't do anything. Seriously, why would they? Someone might have been following him. Someone accidentally tried to open his door. Someone talked to him in a parking garage and gave him a business card. No crimes broken, no threats uttered, etc.
Anyone completely mystified at how someone got past his condo security... shit man, it's easy and happens all the time. Duh.
To the OP: sorry about the others. Sometimes someone on here has some good advice so it is worth asking.
I'll take a shot at understanding what is going on. The parking garage incident may have just been some friendly dude. I guess it's all the vibe you got. But it shouldn't be too hard to verify who he was since you have his card. If it is related then I assume he wanted to know where you were going and how long you'd be gone.
The guy trying to open your door: assuming that this is related to you being followed, then this is the most puzzling part. Any insurance company who would hire a PI to follow you around would strictly want to go by the rules, otherwise risk losing their case on a technicality. Same goes for anyone else either trying to build a case against you or, in some cases, looking for prospects to hire (some companies do a thorough investigation of someone before hiring them).
One more thing: crazy clients don't hire people to follow you around in cheap suits and minivans to see what you're up to.
Is there any chance that someone is investigating one of your clients, looking for dirt on them? But then, why follow you? Unless it's a current case. Just speculating.