I don't think you're being paranoid, but it's a tough situation to call... On one hand, you probably don't want to stay quiet about your suspicions because it won't leave your mind until you ask him about it.
On the other hand, even if he is guilty of doing something he shouldn't have - what do you expect his reaction to be when you confront him?
He'll deny it, and he may even get angry. He's unlikely to confess everything as soon as you accuse him. Instead, he'll get defensive.
The only time the truth will come out about an infidelity is if you have cold hard proof that he simply can't deny, and you simply don't have that here.
There's also the fact that, if you do accuse him and you turn out to be wrong - that it may make things awkward.
A hickey is a pretty obvious mark. Would he be careless enough to let another woman leave a mark on his body for you to see?
Then again, us guys do dumb things like that quite a lot...
Personally, I'd say up the surveillance if you really have doubts. Are there other things he's doing that make you wonder whether there is something going on?
Look for further clues, but also keep a perspective on things.
If things are getting to a boiling point, then maybe you could try the old stake out: Tell him you're going to visit friends or family out of town for a weekend, but instead stick around and keep an eye on what he gets up to. Ask a friend to join you on the stake out and find out for certain what he's doing behind your back!
