I had some one park their car at my house today. I didn't know who's car it was and ultimately had it towed. I looked into the car through the windows but did not enter the car itself to see if it was anyone I know or anyone that my roommates might have known. Turns out it was just a trespasser who didn't have parking permit for the lots next to my house and simply parked at my house without permission.

If I was really concerned of a threat of a trespasser could I have legally searched through the car as long as I didn't do damage or take anything? The car was unlocked, as well.

As some dude on the internet, I would say that yes, if you found a car on your property unlocked, you could search it.

But my question is, why would you?

Are you concerned about there being contraband ie. drugs in there?  If so, I'm not sure you want your fingerprints all over the inside of the vehicle.

Much less if there are any fruits/tools/evidence of a real crime (ie. murder, rape, etc)

It's pretty wild to have some one just park a car in your driveway and leave it. Especially if you don't know who it is. I can deem anything I want that violates my property as a possible threat of a magnitude I can set. Looking into the car, all I saw was just fast food cups and random paper.

It's just a matter if 'I could' instead of 'I would'

What's to stop you?

Search away while wearing latex and make sure someone covers your 6.

Check your local laws

For example
Code of Virginia
§ 18.2-147. Entering or setting in motion, vehicle, aircraft, boat, locomotive or rolling stock of railroad; exceptions.

Any person who shall, without the consent of the owner or person in charge of a vehicle, aircraft, boat, vessel, locomotive or other rolling stock of a railroad, climb into or upon such vehicle, aircraft, boat, vessel, locomotive or other rolling stock of a railroad, with intent to commit any crime, malicious mischief, or injury thereto, or who, while a vehicle, aircraft, boat, vessel, locomotive or other rolling stock of a railroad is at rest and unattended, shall attempt to manipulate any of the levers and starting crank or other device, brakes or mechanism thereof or to set into motion such vehicle, aircraft, boat, vessel, locomotive or other rolling stock of a railroad, with the intent to commit any crime, malicious mischief, or injury thereto, shall be guilty of a Class 1 misdemeanor, except that the foregoing provision shall not apply when any such act is done in an emergency or in furtherance of public safety or by or under the direction of an officer in the regulation of traffic or performance of any other official duty.

So, the law is pretty clear.

1) Don't enter to commit a crime or malicious mischief
2) Don't manipulate the controls that may set the vehicle in motion

Quoted: So, the law is pretty clear. 1) Don't enter to commit a crime or malicious mischief 2) Don't manipulate the controls that may set the vehicle in motion

That does not leave you a lot of reasons to enter the vehicle.
If you remove anything you will have "entered to commit a crime", theft.

Other localities may have different laws that are more or less explicit.

Looking for ownership or anything illegal becomes "theft"?

The OP sounds right in what he could have done.

I don't know if searching would be okay or not.  But I would have had it towed, as well.  Fuck that guy.  And I would NOT tell him who towed it.  I would make him go somewhere else and call all the tow companies to try and find it himself.

Quoted: Looking for ownership or anything illegal becomes "theft"? The OP sounds right in what he could have done.

"My CD, calculator, I-pod, etc. is missing."

You do not need to know ownership to have the vehicle removed in most places.

Do you think anyone tries to see who owns the car when it is towed?

Why would you care?

In Virginia I am even allowed to boot the car if I want, and then charge a fee to remove the boot.

If the car is illegally on MY land, I will use a bulldozer/tractor to push/pull it out onto the road, then let the county tow it.  If it doesnt have far to go, two floor jacks work wonders.

I've towed cars out of my driveway into the middle of the street before, my property, screw 'em

siphon the gas out of the tank before having it towed

Quoted: If the car is illegally on MY land, I will use a bulldozer/tractor to push/pull it out onto the road, then let the county tow it.  If it doesnt have far to go, two floor jacks work wonders.

If you cause damage the vehicle owner can come after you.

Wow, this tread got active.

CONCLUSION!

The owner's father came back the next day after I had it towed. Turns out his was his daughter's car.

Asked me what happened to the car, I said it was parked here illegally, trespassing, and a possible threat to my security and I had it towed. I also hold him to call the police to find who towed it and where. He was pissed, yelled incoherently, and said I was, "inconsiderate."

I moved my car to a friend's on the other side of the block (WITH HER PERMISSION, HA) and let this whole thing cool down for a week. Nothing happened, but I'm still on yellow alert. The times we live in.

Without touching it at all you should have sealed it up using cinder blocks and concrete; putting a brass plaque on it dedicating your "Monument to stupid trespassers" would have been a nice touch.  

Quoted: Wow, this tread got active. CONCLUSION! The owner's father came back the next day after I had it towed. Turns out his was his daughter's car. Asked me what happened to the car, I said it was parked here illegally, trespassing, and a possible threat to my security and I had it towed. I also hold him to call the police to find who towed it and where. He was pissed, yelled incoherently, and said I was, "inconsiderate." I moved my car to a friend's on the other side of the block (WITH HER PERMISSION, HA) and let this whole thing cool down for a week. Nothing happened, but I'm still on yellow alert. The times we live in.

The new America; Where you can do whatever it is you want as long as you have some lame ass excuse that makes other people feel considerate for allowing you to exist in a world without consequences.

Quoted: Wow, this tread got active. CONCLUSION! The owner's father came back the next day after I had it towed. Turns out his was his daughter's car. Asked me what happened to the car, I said it was parked here illegally, trespassing, and a possible threat to my security and I had it towed. I also hold him to call the police to find who towed it and where. He was pissed, yelled incoherently, and said I was, "inconsiderate." I moved my car to a friend's on the other side of the block (WITH HER PERMISSION, HA) and let this whole thing cool down for a week. Nothing happened, but I'm still on yellow alert. The times we live in.

Reply:  You listen to me, you whining fucking cocksucker.  It was inconsiderate to trespass and park in my fucking driveway without my fucking permission.  Now get the fuck out of here.  And tell your cunt little bitch cumdumpster that if she EVER parks here again, I'll use her fucking car for target practice.  NOW FUCK OFF before I throw your ass out of here, you pussy.