Risks of buying vacant lot for new home construction with existing but unused septic system?
Yes, you're being paranoid.

If it was installed correctly, there really isn't much of anything that will go wrong with it from sitting - most septic problems are from using the system and not maintaining it, leading to material that should have been pumped out getting into the drain field and clogging it. If it's not used, that won't happen. The materials do not degrade in the ground. (Orangburg pipe does degrade, but a system built circa 2004 is a good 30+ years removed from the end of Orangeburg - nasty, hateful stuff - basically tar-paper pipes.)

If you keep hunting you can probably find someone who will take your money to run an inspection camera into the pipes. If you like, you could dig up (or open the access manhole, if your area was moving into modern ideas 10 years ago and did not allow the access to be buried below-ground) the septic tank access - but DO NOT stick your head into even an unused septic tank - they can still collect/generate poisonous gases and should aways be treated as if they were, in fact going to kill you if you did that - use a flashlight &amp; mirror on a stick, or hire a pro. If the system is accurately mapped, the distribution box could also be dug down to for a look. If it passed inspection 10 years ago, there's not likely to be much of anything to see in either place, though the tank at least should be looked into (Title V inspection may, in fact, be required before sale, even though it's unused as yet) just to make sure it hasn't shifted or something, and to be sure you know where the access is if it's not above-ground access (consider adding a riser and manhole cover if you have to dig to find the access.)

Don't be surprised if you find it full of water - rainwater seeps in and won't flow out until the liquid inside reaches the outlet level. 10 years allows for quite a bit of rain to fall.

If it was not installed with a particulate filter on the outlet, you might want to have one retrofitted before starting to use it. It may have been - 10 years ago those were starting to become standard in many locales.