I just achieved my commercial drone pilot's license and here is something which may, (or may not) be of help in such a situation.
If you can find out who made the complaint, or by what means, ask to see the evidence (of course all the other information about any "agent" being on your property without premission applies, if you haven't posted your property, be sure to do so asap).
If it appears the "evidence" was via drone, request (aka nicely demand) to see the drone used, and the commercial (Part 107) FAA certificate for whoever was the pilot.
The default basis of flying a drone is, it requires a license... peroid. This is the FAA regulation. However there is a small carveout for recreational flight. That requires several things to be in place, but first and foremost, the flight had to have been undertaken for the sole and exclusive purpose of "recreation". Any ANNY other reason requires an FAA certification as a commercial unmanned aerial system pilot (this even includes for a personal Youtube channel), but especially for real estate aerial photography or videography.
In addition, the drone has to remain in "clear visual line of sight", with unaided vision, meaning, the pilot cannot dip below the treeline, beyond the distance they can actually see the drone (regardless of it's possible range) etc. If at any point they cannot look up and see it, without binoculars, telescope etc, they are beyond visual line of sight and to do that, they would be required to have an FAA approved deviation.
If the drone is used for a commercial reason, any reason, it must be registered with the FAA and have a label on it with the registration number.
Beginning in Sept of 2023, it also must have Radio ID built in, or attached to it.
If any of this cannot be proven by the pilot, the flight is illegal, and any "evidence" from the flight.. well, that's where an attorney might come in useful.
