Steps after recovering my lost/stolen iphone?

Earlier today, my iPhone 4 went missing. I believe I dropped it while running errands, most likely at Walmart. I didn't realize until later that my phone was missing, until I got home. From there, I called my iPhone, and a person answered who said that he'd found my iPhone on the ground several miles from where I live and where I was running errands. He said that he found it on the ground, with the white apple logo on the screen; he said that maybe somebody restored it.

When I was able to get my iPhone back, it was indeed restored/wiped. My question is, do I need to take any precautionary steps after this incident? I only have my email information on my phone, and no credit card or other payment information.

Should I restore again from my computer? Should I contact my mobile provider (no calls were made, however, when it was missing)? What's the worst that can happen?

I would change your email passwords immediately. Check with your email account for a recent history. You will likely be able to see when it was accessed recently and from what ip addresses at least. If anyone who has accessed it wasn't you, then you need to start thinking about what is in your email account. Could someone access your bank account, change or recover sensitive passwords from the email? You may have more accounts you need to change info on.

As far as no calls being made, how do you know if it was factory reset? That would erase all personal data and all history. I would be wondering why someone would reset it, unless to cover up what they had been doing while the phone was in their possession.