I am not an expert on ED billing rules, but per my reading of the Medicare regulations, this would not meet the definition of a Medicare covered service. Per the Medicare Benefit Policy Manual, in order to be covered, outpatient "services and supplies must be furnished as an integral, although incidental, part of the physician or nonphysician practitioner’s professional service in the course of treatment of an illness or injury."
It goes on to explain that: "services and supplies must be furnished under the order of a physician or other practitioner practicing within the extent of the Act, the Code of Federal Regulations, and State law. They must be furnished by hospital personnel under the appropriate supervision of a physician or nonphysician practitioner as required in this manual and by 42 CFR 410.27 and 482.12. This does not mean that each occasion of service by a nonphysician need also be the occasion of the actual rendition of a personal professional service by the physician responsible for care of the patient. However, during any course of treatment rendered by auxiliary personnel, the physician must personally see the patient periodically and sufficiently often to assess the course of treatment and the patient’s progress and, when necessary, to change the treatment regimen. A hospital service or supply would not be considered incident to a physician’s service if the attending physician merely wrote an order for the services or supplies and referred the patient to the hospital without being involved in the management of that course of treatment."
Unless Medicare has published an exception to this rule that I'm not aware of, by my reading of this if a patient presents to an ED and leaves before being evaluated by a physician or without direct physician involvement in the triage, this is not a covered service and would not be appropriate to bill to Medicare with either 99281 or 99211.