Does a failed dental implant due to bone loss always mean there's infection? and can infection be seen on x-ray?

No and No: Bone loss can occur for many reasons, some of which are not related to infection. Radiographs do not show an infection... they image different densities in bone and teeth. Less dense bone or tooth structure indicates decalcification (which is usually caused by bacteria), but you cannot literally see the infection. Inflammation can also cause bone loss.

Tricky Question: Infection cannot be seen on an X-ray....even a 3D scan. Only the bone loss itself can be seen. Bone loss is the result of local inflammation. Often, this is caused by an infection (bacteria). However, it can also be caused by bite issues or trauma (no bacteria). Sometimes, we don't know where the inflammation came from...environment, metabolism, hormones, nutrition, combination, etc.???