This is normal if the camera is set to continuous file numbering. Canon EOS bodies can continue the image number sequence from the highest number seen previously, including from another memory card.

In your case, the likely explanation is exactly that: a different card with images up to IMG_9544 had been inserted earlier, so the 80D continued at IMG_9545 even when you later used a newly formatted card.

So the filename alone does not prove the camera was used. File numbering is separate from whether the camera is new.

If you want, you can change the camera’s file numbering behavior or manually reset it in the 80D menu so new images start over from 0001 in a new folder. If you’re worried about whether the camera itself was previously used, shutter count is a better indicator than filename numbering.