The question probably boils down to whether or not such practice is within (or outside of) the generally accepted "duty of care" owed by reasonably prudent physicians in the same (or similar) areas of practice, degree of training/experience, geographic locale, etc.

For example, if other patient records are known to exist and relatively easy to procure (e.g., as kept by another department within the same facility), probably there would be a duty to obtain/consider them as relevant to the current condition. Failure to do so could thus be deemed a breach of the generally accepted duty of care. If, on the other hand, patient records might only potentially exist at, say, an unrelated facility very far away - and there is no routine procedure for sharing patient records with that facility - failing to obtain them could well be deemed reasonable and within the duty of care. (In the latter example, it is still possible, however, that the facility’s overarching failure to establish procedures for obtaining such records could be deemed negligent in/of itself, especially if record-sharing procedures are usually in place at similar local facilities.) Cases between these two examples would turn on their specific facts as measured against the usual duty of care.

Looking into how physicians at your facility and those surrounding it handle this situation might be a good idea. I also recommend keeping a close eye on the development of electronic patient records, which would presumably be more easily obtainable, and could thus impose a duty to obtain/review them, especially if your peers generally do.