I suggest you stop trying to figure out why. It doesn't sound like anything is happening that would cause a PI to investigate. And, I would be cautious but not frightened by this if this has only happened once.
I've had people take pictures of my house. I've taken pictures of yards. I would assume that this is not serious but be more aware for the next couple of weeks. If it doesn't happen again, I'd let it go.
Get the license number of any suspicious cars and call the police as they've suggested. And if you think someone is following you do not go home. Go to a public place where there are lots of people. Perhaps drive around a neighborhood making frequent turns to see if they continue to follow you.
For this instance, it sounds like this person, if he was interested in you, already knew where you live. He was waiting for you. But please consider that this could all be coincidental. I suggest that if he's a PI he's interested in a neighbor and not you.
To put to rest one of the ideas about reasons to be followed. Insurance would only be investigating your activities when you make a claim for disability and have reason to believe that you might be faking it. Your injury is recent and you're receiving medical care. Nothing to investigate there.
Also, if he were investigating you and a physical disability he would not follow you this closely. His goal would be to discover how you functioned. You could fake it if you knew he were watching. I suggest that this has a logical explanation and is not a safety issue for you. Just be watchful to be sure.