EC135 helicopter Captain reported an engine malfunction requiring a landing precaution on a taxiway on an ambulance flight prior to Tower operating hours. The pilot remained with the aircraft until company maintenance arrived then walked on an active taxiway to the FBO; resulting in a pedestrian violation from ATC once the Tower opened.

Narrative

At approximately XA:30 I landed my aircraft on the Taxiway 1 Northeast of the FBO/GA ramp at ZZZ during a flight due to a malfunctioning oil cooling system on the #1 engine. Landing was uneventful and all required company procedures were followed post landing. A representative of Airport Operations met us on the taxiway and after we explained the situation and the need for a ground ambulance to be escorted out to transfer the patient he departed and eventually returned escorting the ambulance. Once the ambulance had departed he asked how long I would be disabled on the taxiway. I indicated that it would most likely be several hours until maintainers would arrive with ground handling equipment and he stated that he would close the taxiway and have a NOTAM issued closing it. I indicated that I wished to remain with the aircraft until the maintainers arrived and he indicated his approval for me to do so with no further discussion.I expected at some point for this person to return with bean bag lights or physical barriers to indicate the taxiway closure or to mark my aircraft on the taxiway; but that did not happen. Part of my desire to remain with the aircraft; in addition to guarding it per Company policy; was I was concerned that a NOTAM would not prevent an aircraft from entering the taxiway and possibly colliding with it the dark. My thought was I could turn on the aircraft exterior lighting in the event an aircraft approached. I remained in the aircraft until XC:04; at which time there was sufficient natural light to identify the aircraft on the taxiway. Also at that time I felt the need to urinate; so I exited the aircraft and walked a short distance south on the Taxiway 1 to the GA Ramp to the FBO to use the restroom. Upon returning to the lobby the FBO personnel indicated the Tower had called and asked me to call them. I called Tower and the Controller on Duty indicated that I had committed a Pedestrian Violation by walking on an active taxiway. I informed him of the conversation I had with the representative of Airfield Operations of the taxiway closure and NOTAM and the Controller indicated that neither had occurred and that he was unaware I was in the aircraft until he saw me walking on the taxiway. I obtained his permission to return to the aircraft by not using the taxiway; but instead walking along the non-movement area until abeam my aircraft; then walking across the sod directly to it; because he considered the taxiway to still be active. I remained there until the maintainers arrived at the FBO; then I called the tower and obtained permission to exit back to the FBO using the reverse procedure walking back across the sod to the non movement area.When I initially exited my aircraft I was under the impression that the taxiway was NOTAM closed. In hindsight; I should have realized the tower was open at XC:00 and called the tower; but at the time I did think that was necessary as I had been told the taxiway was closed; a NOTAM issued and that all relevant airport personnel were aware of my presence. During our initial conversation the Controller indicated that if I had exited my aircraft prior to XC:00 he would not have issued the Pedestrian Violation since the Tower was not open and the only reason he was issuing it was because he considered the taxiway to still be active at the time I walked to the FBO; and after; despite my coordination with Airfield Operations.